readwrite.c

Go to the documentation of this file.

/*
 *  ReactOS Floppy Driver
 *  Copyright (C) 2004, Vizzini (vizzini@plasmic.com)
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * PROJECT:         ReactOS Floppy Driver
 * FILE:            readwrite.c
 * PURPOSE:         Read/Write handler routines
 * PROGRAMMER:      Vizzini (vizzini@plasmic.com)
 * REVISIONS:
 *                  15-Feb-2004 vizzini - Created
 * NOTES:
 *
 * READ/WRITE PROCESS
 *
 * This process is extracted from the Intel datasheet for the floppy controller.
 *
 * - Turn on the motor and set turnoff time
 * - Program the drive's data rate
 * - Seek
 * - Read ID
 * - Set up DMA
 * - Send read/write command to FDC
 * - Read result bytes
 *
 * This is mostly implemented in one big function, which watis on the SynchEvent
 * as many times as necessary to get through the process.  See ReadWritePassive() for
 * more details.
 *
 * NOTES:
 *     - Currently doesn't support partial-sector transfers, which is really just a failing
 *       of RWComputeCHS.  I've never seen Windows send a partial-sector request, though, so
 *       this may not be a bad thing.  Should be looked into, regardless.
 *
 * TODO: Break up ReadWritePassive and handle errors better
 * TODO: Figure out data rate issues
 * TODO: Media type detection
 * TODO: Figure out perf issue - waiting after call to read/write for about a second each time
 * TODO: Figure out specify timings
 */

#include "precomp.h"


static IO_ALLOCATION_ACTION NTAPI
MapRegisterCallback(PDEVICE_OBJECT DeviceObject,
                    PIRP Irp,
                    PVOID MapRegisterBase,
                    PVOID Context)
/*
 * FUNCTION: Acquire map registers in prep for DMA
 * ARGUMENTS:
 *     DeviceObject: unused
 *     Irp: unused
 *     MapRegisterBase: returned to blocked thread via a member var
 *     Context: contains a pointer to the right ControllerInfo
 *     struct
 * RETURNS:
 *     KeepObject, because that's what the DDK says to do
 */
{
    PCONTROLLER_INFO ControllerInfo = (PCONTROLLER_INFO)Context;
    UNREFERENCED_PARAMETER(DeviceObject);
    UNREFERENCED_PARAMETER(Irp);

    TRACE_(FLOPPY, "MapRegisterCallback Called\n");

    ControllerInfo->MapRegisterBase = MapRegisterBase;
    KeSetEvent(&ControllerInfo->SynchEvent, 0, FALSE);

    return KeepObject;
}


NTSTATUS NTAPI
ReadWrite(PDEVICE_OBJECT DeviceObject, PIRP Irp)
/*
 * FUNCTION: Dispatch routine called for read or write IRPs
 * ARGUMENTS:
 * RETURNS:
 *     STATUS_PENDING if the IRP is queued
 *     STATUS_INVALID_PARAMETER if IRP is set up wrong
 * NOTES:
 *     - This function validates arguments to the IRP and then queues it
 *     - Note that this function is implicitly serialized by the queue logic.  Only
 *       one of these at a time is active in the system, no matter how many processors
 *       and threads we have.
 *     - This function stores the DeviceObject in the IRP's context area before dropping
 *       it onto the irp queue
 */
{
    TRACE_(FLOPPY, "ReadWrite called\n");

    ASSERT(DeviceObject);
    ASSERT(Irp);

    if(!Irp->MdlAddress)
    {
        WARN_(FLOPPY, "ReadWrite(): MDL not found in IRP - Completing with STATUS_INVALID_PARAMETER\n");
        Irp->IoStatus.Status = STATUS_INVALID_PARAMETER;
        Irp->IoStatus.Information = 0;
        IoCompleteRequest(Irp, IO_NO_INCREMENT);
        return STATUS_INVALID_PARAMETER;
    }

    /*
     * Queue the irp to the thread.
     * The de-queue thread will look in DriverContext[0] for the Device Object.
     */
    Irp->Tail.Overlay.DriverContext[0] = DeviceObject;
    IoCsqInsertIrp(&Csq, Irp, NULL);

    return STATUS_PENDING;
}


static VOID NTAPI
RWFreeAdapterChannel(PADAPTER_OBJECT AdapterObject)
/*
 * FUNCTION: Free the adapter DMA channel that we allocated
 * ARGUMENTS:
 *     AdapterObject: the object with the map registers to free
 * NOTES:
 *     - This function is primarily needed because IoFreeAdapterChannel wants to
 *       be called at DISPATCH_LEVEL
 */
{
    KIRQL Irql;

    ASSERT(KeGetCurrentIrql() <= DISPATCH_LEVEL);

    KeRaiseIrql(DISPATCH_LEVEL, &Irql);
    IoFreeAdapterChannel(AdapterObject);
    KeLowerIrql(Irql);
}


static NTSTATUS NTAPI
RWDetermineMediaType(PDRIVE_INFO DriveInfo)
/*
 * FUNCTION: Determine the media type of the disk in the drive and fill in the geometry
 * ARGUMENTS:
 *     DriveInfo: drive to look at
 * RETURNS:
 *     STATUS_SUCCESS if the media was recognized and the geometry struct was filled in
 *     STATUS_UNRECOGNIZED_MEDIA if not
 *     STATUS_UNSUCCESSFUL if the controller can't be talked to
 * NOTES:
 *     - Expects the motor to already be running
 *     - Currently only supports 1.44MB 3.5" disks
 *     - PAGED_CODE because it waits
 * TODO:
 *     - Support more disk types
 */
{
    UCHAR HeadLoadTime;
    UCHAR HeadUnloadTime;
    UCHAR StepRateTime;

    PAGED_CODE();

    TRACE_(FLOPPY, "RWDetermineMediaType called\n");

    /*
     * This algorithm assumes that a 1.44MB floppy is in the drive.  If it's not,
     * it works backwards until the read works.  Note that only 1.44 has been tested
     * at all.
     */

    do
    {
        int i;

        /* Program data rate */
        if(HwSetDataRate(DriveInfo->ControllerInfo, DRSR_DSEL_500KBPS) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "RWDetermineMediaType(): unable to set data rate\n");
            return STATUS_UNSUCCESSFUL;
        }

        /* Specify */
        HeadLoadTime = SPECIFY_HLT_500K;
        HeadUnloadTime = SPECIFY_HUT_500K;
        StepRateTime = SPECIFY_SRT_500K;

        /* Don't disable DMA --> enable dma (dumb & confusing) */
        if(HwSpecify(DriveInfo->ControllerInfo, HeadLoadTime, HeadUnloadTime, StepRateTime, FALSE) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "RWDetermineMediaType(): specify failed\n");
            return STATUS_UNSUCCESSFUL;
        }

        /* clear any spurious interrupts in preparation for recalibrate */
        KeClearEvent(&DriveInfo->ControllerInfo->SynchEvent);

        /* Recalibrate --> head over first track */
        for(i=0; i < 2; i++)
        {
            NTSTATUS RecalStatus;

            if(HwRecalibrate(DriveInfo) != STATUS_SUCCESS)
            {
                WARN_(FLOPPY, "RWDetermineMediaType(): Recalibrate failed\n");
                return STATUS_UNSUCCESSFUL;
            }

            /* Wait for the recalibrate to finish */
            WaitForControllerInterrupt(DriveInfo->ControllerInfo);

            RecalStatus = HwRecalibrateResult(DriveInfo->ControllerInfo);

            if(RecalStatus == STATUS_SUCCESS)
                break;

            if(i == 1) /* failed for 2nd time */
            {
                WARN_(FLOPPY, "RWDetermineMediaType(): RecalibrateResult failed\n");
                return STATUS_UNSUCCESSFUL;
            }
        }

        /* clear any spurious interrupts */
        KeClearEvent(&DriveInfo->ControllerInfo->SynchEvent);

        /* Try to read an ID */
        if(HwReadId(DriveInfo, 0) != STATUS_SUCCESS) /* read the first ID we find, from head 0 */
        {
            WARN_(FLOPPY, "RWDetermineMediaType(): ReadId failed\n");
            return STATUS_UNSUCCESSFUL; /* if we can't even write to the controller, it's hopeless */
        }

        /* Wait for the ReadID to finish */
        WaitForControllerInterrupt(DriveInfo->ControllerInfo);

        if(HwReadIdResult(DriveInfo->ControllerInfo, NULL, NULL) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "RWDetermineMediaType(): ReadIdResult failed; continuing\n");
            continue;
        }

        /* Found the media; populate the geometry now */
        WARN_(FLOPPY, "Hardcoded media type!\n");
        INFO_(FLOPPY, "RWDetermineMediaType(): Found 1.44 media; returning success\n");
        DriveInfo->DiskGeometry.MediaType = GEOMETRY_144_MEDIATYPE;
        DriveInfo->DiskGeometry.Cylinders.QuadPart = GEOMETRY_144_CYLINDERS;
        DriveInfo->DiskGeometry.TracksPerCylinder = GEOMETRY_144_TRACKSPERCYLINDER;
        DriveInfo->DiskGeometry.SectorsPerTrack = GEOMETRY_144_SECTORSPERTRACK;
        DriveInfo->DiskGeometry.BytesPerSector = GEOMETRY_144_BYTESPERSECTOR;
        DriveInfo->BytesPerSectorCode = HW_512_BYTES_PER_SECTOR;
        return STATUS_SUCCESS;
    }
    while(FALSE);

    TRACE_(FLOPPY, "RWDetermineMediaType(): failed to find media\n");
    return STATUS_UNRECOGNIZED_MEDIA;
}


static NTSTATUS NTAPI
RWSeekToCylinder(PDRIVE_INFO DriveInfo, UCHAR Cylinder)
/*
 * FUNCTION: Seek a particular drive to a particular track
 * ARGUMENTS:
 *     DriveInfo: Drive to seek
 *     Cylinder: track to seek to
 * RETURNS:
 *     STATUS_SUCCESS if the head was successfully seeked
 *     STATUS_UNSUCCESSFUL if not
 * NOTES:
 *     - PAGED_CODE because it blocks
 */
{
    UCHAR CurCylinder;

    PAGED_CODE();

    TRACE_(FLOPPY, "RWSeekToCylinder called drive 0x%p cylinder %d\n", DriveInfo, Cylinder);

    /* Clear any spurious interrupts */
    KeClearEvent(&DriveInfo->ControllerInfo->SynchEvent);

    /* queue seek command */
    if(HwSeek(DriveInfo, Cylinder) != STATUS_SUCCESS)
    {
        WARN_(FLOPPY, "RWSeekToTrack(): unable to seek\n");
        return STATUS_UNSUCCESSFUL;
    }

    WaitForControllerInterrupt(DriveInfo->ControllerInfo);

    if(HwSenseInterruptStatus(DriveInfo->ControllerInfo) != STATUS_SUCCESS)
    {
        WARN_(FLOPPY, "RWSeekToTrack(): unable to get seek results\n");
        return STATUS_UNSUCCESSFUL;
    }

    /* read ID mark from head 0 to verify */
    if(HwReadId(DriveInfo, 0) != STATUS_SUCCESS)
    {
        WARN_(FLOPPY, "RWSeekToTrack(): unable to queue ReadId\n");
        return STATUS_UNSUCCESSFUL;
    }

    WaitForControllerInterrupt(DriveInfo->ControllerInfo);

    if(HwReadIdResult(DriveInfo->ControllerInfo, &CurCylinder, NULL) != STATUS_SUCCESS)
    {
        WARN_(FLOPPY, "RWSeekToTrack(): unable to get ReadId result\n");
        return STATUS_UNSUCCESSFUL;
    }

    if(CurCylinder != Cylinder)
    {
        WARN_(FLOPPY, "RWSeekToTrack(): Seeek to track failed; current cylinder is 0x%x\n", CurCylinder);
        return STATUS_UNSUCCESSFUL;
    }

    INFO_(FLOPPY, "RWSeekToCylinder: returning successfully, now on cyl %d\n", Cylinder);

    return STATUS_SUCCESS;
}


static NTSTATUS NTAPI
RWComputeCHS(PDRIVE_INFO IN  DriveInfo,
             ULONG       IN  DiskByteOffset,
             PUCHAR      OUT Cylinder,
             PUCHAR      OUT Head,
             PUCHAR      OUT Sector)
/*
 * FUNCTION: Compute the CHS from the absolute byte offset on disk
 * ARGUMENTS:
 *     DriveInfo: Drive to compute on
 *     DiskByteOffset: Absolute offset on disk of the starting byte
 *     Cylinder: Cylinder that the byte is on
 *     Head: Head that the byte is on
 *     Sector: Sector that the byte is on
 * RETURNS:
 *     STATUS_SUCCESS if CHS are determined correctly
 *     STATUS_UNSUCCESSFUL otherwise
 * NOTES:
 *     - Lots of ugly typecasts here
 *     - Sectors are 1-based!
 *     - This is really crummy code.  Please FIXME.
 */
{
    ULONG AbsoluteSector;
    UCHAR SectorsPerCylinder = (UCHAR)DriveInfo->DiskGeometry.SectorsPerTrack * (UCHAR)DriveInfo->DiskGeometry.TracksPerCylinder;

    TRACE_(FLOPPY, "RWComputeCHS: Called with offset 0x%x\n", DiskByteOffset);

    /* First calculate the 1-based "absolute sector" based on the byte offset */
    ASSERT(!(DiskByteOffset % DriveInfo->DiskGeometry.BytesPerSector));         /* FIXME: Only handle full sector transfers atm */

    /* AbsoluteSector is zero-based to make the math a little easier */
    AbsoluteSector = DiskByteOffset / DriveInfo->DiskGeometry.BytesPerSector;  /* Num full sectors */

    /* Cylinder number is floor(AbsoluteSector / SectorsPerCylinder) */
    *Cylinder =  (CHAR)(AbsoluteSector / SectorsPerCylinder);

    /* Head number is 0 if the sector within the cylinder < SectorsPerTrack; 1 otherwise */
    *Head =  AbsoluteSector % SectorsPerCylinder < DriveInfo->DiskGeometry.SectorsPerTrack ? 0 : 1;

    /*
     * Sector number is the sector within the cylinder if on head 0; that minus SectorsPerTrack if it's on head 1
     * (lots of casts to placate msvc).  1-based!
     */
    *Sector =  ((UCHAR)(AbsoluteSector % SectorsPerCylinder) + 1) - ((*Head) * (UCHAR)DriveInfo->DiskGeometry.SectorsPerTrack);

    INFO_(FLOPPY, "RWComputeCHS: offset 0x%x is c:0x%x h:0x%x s:0x%x\n", DiskByteOffset, *Cylinder, *Head, *Sector);

    /* Sanity checking */
    ASSERT(*Cylinder <= DriveInfo->DiskGeometry.Cylinders.QuadPart);
    ASSERT(*Head <= DriveInfo->DiskGeometry.TracksPerCylinder);
    ASSERT(*Sector <= DriveInfo->DiskGeometry.SectorsPerTrack);

    return STATUS_SUCCESS;
}


VOID NTAPI
ReadWritePassive(PDRIVE_INFO DriveInfo, PIRP Irp)
/*
 * FUNCTION: Handle the first phase of a read or write IRP
 * ARGUMENTS:
 *     DeviceObject: DeviceObject that is the target of the IRP
 *     Irp: IRP to process
 * RETURNS:
 *     STATUS_VERIFY_REQUIRED if the media has changed and we need the filesystems to re-synch
 *     STATUS_SUCCESS otherwise
 * NOTES:
 *     - Must be called at PASSIVE_LEVEL
 *     - This function is about 250 lines longer than I wanted it to be.  Sorry.
 *
 * DETAILS:
 *  This routine manages the whole process of servicing a read or write request.  It goes like this:
 *    1) Check the DO_VERIFY_VOLUME flag and return if it's set
 *    2) Check the disk change line and notify the OS if it's set and return
 *    3) Detect the media if we haven't already
 *    4) Set up DiskByteOffset, Length, and WriteToDevice parameters
 *    5) Get DMA map registers
 *    6) Then, in a loop for each track, until all bytes are transferred:
 *      a) Compute the current CHS to set the read/write head to
 *      b) Seek to that spot
 *      c) Compute the last sector to transfer on that track
 *      d) Map the transfer through DMA
 *      e) Send the read or write command to the controller
 *      f) Read the results of the command
 */
{
    PDEVICE_OBJECT DeviceObject = DriveInfo->DeviceObject;
    PIO_STACK_LOCATION Stack = IoGetCurrentIrpStackLocation(Irp);
    BOOLEAN WriteToDevice;
    ULONG Length;
    ULONG DiskByteOffset;
    KIRQL OldIrql;
    NTSTATUS Status;
    BOOLEAN DiskChanged;
    ULONG_PTR TransferByteOffset;
    UCHAR Gap;

    PAGED_CODE();

    TRACE_(FLOPPY, "ReadWritePassive called to %s 0x%x bytes from offset 0x%x\n",
           (Stack->MajorFunction == IRP_MJ_READ ? "read" : "write"),
           (Stack->MajorFunction == IRP_MJ_READ ? Stack->Parameters.Read.Length : Stack->Parameters.Write.Length),
           (Stack->MajorFunction == IRP_MJ_READ ? Stack->Parameters.Read.ByteOffset.u.LowPart :
            Stack->Parameters.Write.ByteOffset.u.LowPart));

    /* Default return codes */
    Irp->IoStatus.Status = STATUS_UNSUCCESSFUL;
    Irp->IoStatus.Information = 0;

    /*
     * Check to see if the volume needs to be verified.  If so,
     * we can get out of here quickly.
     */
    if(DeviceObject->Flags & DO_VERIFY_VOLUME && !(Stack->Flags & SL_OVERRIDE_VERIFY_VOLUME))
    {
        INFO_(FLOPPY, "ReadWritePassive(): DO_VERIFY_VOLUME set; Completing with  STATUS_VERIFY_REQUIRED\n");
        Irp->IoStatus.Status = STATUS_VERIFY_REQUIRED;
        IoCompleteRequest(Irp, IO_NO_INCREMENT);
        return;
    }

    /*
     * Check the change line, and if it's set, return
     */
    StartMotor(DriveInfo);
    if(HwDiskChanged(DeviceObject->DeviceExtension, &DiskChanged) != STATUS_SUCCESS)
    {
        WARN_(FLOPPY, "ReadWritePassive(): unable to detect disk change; Completing with STATUS_UNSUCCESSFUL\n");
        IoCompleteRequest(Irp, IO_NO_INCREMENT);
        StopMotor(DriveInfo->ControllerInfo);
        return;
    }

    if(DiskChanged)
    {
        INFO_(FLOPPY, "ReadWritePhase1(): signalling media changed; Completing with STATUS_MEDIA_CHANGED\n");

        /* The following call sets IoStatus.Status and IoStatus.Information */
        SignalMediaChanged(DeviceObject, Irp);

        /*
         * Guessing at something... see ioctl.c for more info
         */
        if(ResetChangeFlag(DriveInfo) == STATUS_NO_MEDIA_IN_DEVICE)
            Irp->IoStatus.Status = STATUS_NO_MEDIA_IN_DEVICE;

        IoCompleteRequest(Irp, IO_NO_INCREMENT);
        StopMotor(DriveInfo->ControllerInfo);
        return;
    }

    /*
     * Figure out the media type, if we don't know it already
     */
    if(DriveInfo->DiskGeometry.MediaType == Unknown)
    {
        if(RWDetermineMediaType(DriveInfo) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "ReadWritePassive(): unable to determine media type; completing with STATUS_UNSUCCESSFUL\n");
            IoCompleteRequest(Irp, IO_NO_INCREMENT);
            StopMotor(DriveInfo->ControllerInfo);
            return;
        }

        if(DriveInfo->DiskGeometry.MediaType == Unknown)
        {
            WARN_(FLOPPY, "ReadWritePassive(): Unknown media in drive; completing with STATUS_UNRECOGNIZED_MEDIA\n");
            Irp->IoStatus.Status = STATUS_UNRECOGNIZED_MEDIA;
            IoCompleteRequest(Irp, IO_NO_INCREMENT);
            StopMotor(DriveInfo->ControllerInfo);
            return;
        }
    }

    /* Set up parameters for read or write */
    if(Stack->MajorFunction == IRP_MJ_READ)
    {
        Length = Stack->Parameters.Read.Length;
        DiskByteOffset = Stack->Parameters.Read.ByteOffset.u.LowPart;
        WriteToDevice = FALSE;
    }
    else
    {
        Length = Stack->Parameters.Write.Length;
        DiskByteOffset = Stack->Parameters.Write.ByteOffset.u.LowPart;
        WriteToDevice = TRUE;
    }

    /*
     * FIXME:
     *   FloppyDeviceData.ReadWriteGapLength specify the value for the physical drive.
     *   We should set this value depend on the format of the inserted disk and possible
     *   depend on the request (read or write). A value of 0 results in one rotation
     *   between the sectors (7.2sec for reading a track).
     */
    Gap = DriveInfo->FloppyDeviceData.ReadWriteGapLength;

    /*
     * Set up DMA transfer
     *
     * This is as good of a place as any to document something that used to confuse me
     * greatly (and I even wrote some of the kernel's DMA code, so if it confuses me, it
     * probably confuses at least a couple of other people too).
     *
     * MmGetMdlVirtualAddress() returns the virtal address, as mapped in the buffer's original
     * process context, of the MDL.  In other words:  say you start with a buffer at address X, then
     * you build an MDL out of that buffer called Mdl. If you call MmGetMdlVirtualAddress(Mdl), it
     * will return X.
     *
     * There are two parameters that the function looks at to produce X again, given the MDL:  the
     * first is the StartVa, which is the base virtual address of the page that the buffer starts
     * in.  If your buffer's virtual address is 0x12345678, StartVa will be 0x12345000, assuming 4K pages
     * (which is (almost) always the case on x86).  Note well: this address is only valid in the
     * process context that you initially built the MDL from.  The physical pages that make up
     * the MDL might perhaps be mapped in other process contexts too (or even in the system space,
     * above 0x80000000 (default; 0xc0000000 on current ReactOS or /3GB Windows)), but it will
     * (possibly) be mapped at a different address.
     *
     * The second parameter is the ByteOffset.  Given an original buffer address of 0x12345678,
     * the ByteOffset would be 0x678.  Because MDLs can only describe full pages (and therefore
     * StartVa always points to the start address of a page), the ByteOffset must be used to
     * find the real start of the buffer.
     *
     * In general, if you add the StartVa and ByteOffset together, you get back your original
     * buffer pointer, which you are free to use if you're sure you're in the right process
     * context.  You could tell by accessing the (hidden and not-to-be-used) Process member of
     * the MDL, but in general, if you have to ask whether or not you are in the right context,
     * then you shouldn't be using this address for anything anyway.  There are also security implications
     * (big ones, really, I wouldn't kid about this) to directly accessing a user's buffer by VA, so
     * Don't Do That.
     *
     * There is a somewhat weird but very common use of the virtual address associated with a MDL
     * that pops up often in the context of DMA.  DMA APIs (particularly MapTransfer()) need to
     * know where the memory is that they should DMA into and out of.  This memory is described
     * by a MDL.  The controller eventually needs to know a physical address on the host side,
     * which is generally a 32-bit linear address (on x86), and not just a page address.  Therefore,
     * the DMA APIs look at the ByteOffset field of the MDL to reconstruct the real address that
     * should be programmed into the DMA controller.
     *
     * It is often the case that a transfer needs to be broken down over more than one DMA operation,
     * particularly when it is a big transfer and the HAL doesn't give you enough map registers
     * to map the whole thing at once.  Therefore, the APIs need a way to tell how far into the MDL
     * they should look to transfer the next chunk of bytes.  Now, Microsoft could have designed
     * MapTransfer to take a  "MDL offset" argument, starting with 0, for how far into the buffer to
     * start, but it didn't.  Instead, MapTransfer asks for the virtual address of the MDL as an "index" into
     * the MDL.  The way it computes how far into the page to start the transfer is by masking off all but
     * the bottom 12 bits (on x86) of the number you supply as the CurrentVa and using *that* as the
     * ByteOffset instead of the one in the MDL.  (OK, this varies a bit by OS and version, but this
     * is the effect).
     *
     * In other words, you get a number back from MmGetMdlVirtualAddress that represents the start of your
     * buffer, and you pass it to the first MapTransfer call.  Then, for each successive operation
     * on the same buffer, you increment that address to point to the next spot in the MDL that
     * you want to DMA to/from.  The fact that the virtual address you're manipulating is probably not
     * mapped into the process context that you're running in is irrelevant, since it's only being
     * used to index into the MDL.
     */

    /* Get map registers for DMA */
    KeRaiseIrql(DISPATCH_LEVEL, &OldIrql);
    Status = IoAllocateAdapterChannel(DriveInfo->ControllerInfo->AdapterObject, DeviceObject,
                                      DriveInfo->ControllerInfo->MapRegisters, MapRegisterCallback, DriveInfo->ControllerInfo);
    KeLowerIrql(OldIrql);

    if(Status != STATUS_SUCCESS)
    {
        WARN_(FLOPPY, "ReadWritePassive(): unable allocate an adapter channel; completing with STATUS_UNSUCCESSFUL\n");
        IoCompleteRequest(Irp, IO_NO_INCREMENT);
        StopMotor(DriveInfo->ControllerInfo);
        return ;
    }


    /*
     * Read from (or write to) the device
     *
     * This has to be called in a loop, as you can only transfer data to/from a single track at
     * a time.
     */
    TransferByteOffset = 0;
    while(TransferByteOffset < Length)
    {
        UCHAR Cylinder;
        UCHAR Head;
        UCHAR StartSector;
        ULONG CurrentTransferBytes;
        UCHAR CurrentTransferSectors;

        INFO_(FLOPPY, "ReadWritePassive(): iterating in while (TransferByteOffset = 0x%x of 0x%x total) - allocating %d registers\n",
              TransferByteOffset, Length, DriveInfo->ControllerInfo->MapRegisters);

        KeClearEvent(&DriveInfo->ControllerInfo->SynchEvent);

        /*
         * Compute starting CHS
         */
        if(RWComputeCHS(DriveInfo, DiskByteOffset+TransferByteOffset, &Cylinder, &Head, &StartSector) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "ReadWritePassive(): unable to compute CHS; completing with STATUS_UNSUCCESSFUL\n");
            RWFreeAdapterChannel(DriveInfo->ControllerInfo->AdapterObject);
            IoCompleteRequest(Irp, IO_NO_INCREMENT);
            StopMotor(DriveInfo->ControllerInfo);
            return;
        }

        /*
         * Seek to the right track
         */
        if(!DriveInfo->ControllerInfo->ImpliedSeeks)
        {
            if(RWSeekToCylinder(DriveInfo, Cylinder) != STATUS_SUCCESS)
            {
                WARN_(FLOPPY, "ReadWritePassive(): unable to seek; completing with STATUS_UNSUCCESSFUL\n");
                RWFreeAdapterChannel(DriveInfo->ControllerInfo->AdapterObject);
                IoCompleteRequest(Irp, IO_NO_INCREMENT);
                StopMotor(DriveInfo->ControllerInfo);
                return ;
            }
        }

        /*
         * Compute last sector
         *
         * We can only ask for a transfer up to the end of the track.  Then we have to re-seek and do more.
         * TODO: Support the MT bit
         */
        INFO_(FLOPPY, "ReadWritePassive(): computing number of sectors to transfer (StartSector 0x%x): ", StartSector);

        /* 1-based sector number */
        if( (((DriveInfo->DiskGeometry.TracksPerCylinder - Head) * DriveInfo->DiskGeometry.SectorsPerTrack - StartSector) + 1 ) <
                (Length - TransferByteOffset) / DriveInfo->DiskGeometry.BytesPerSector)
        {
            CurrentTransferSectors = (UCHAR)((DriveInfo->DiskGeometry.TracksPerCylinder - Head) * DriveInfo->DiskGeometry.SectorsPerTrack - StartSector) + 1;
        }
        else
        {
            CurrentTransferSectors = (UCHAR)((Length - TransferByteOffset) / DriveInfo->DiskGeometry.BytesPerSector);
        }

        INFO_(FLOPPY, "0x%x\n", CurrentTransferSectors);

        CurrentTransferBytes = CurrentTransferSectors * DriveInfo->DiskGeometry.BytesPerSector;

        /*
         * Adjust to map registers
         * BUG: Does this take into account page crossings?
         */
        INFO_(FLOPPY, "ReadWritePassive(): Trying to transfer 0x%x bytes\n", CurrentTransferBytes);

        ASSERT(CurrentTransferBytes);

        if(BYTES_TO_PAGES(CurrentTransferBytes) > DriveInfo->ControllerInfo->MapRegisters)
        {
            CurrentTransferSectors = (UCHAR)((DriveInfo->ControllerInfo->MapRegisters * PAGE_SIZE) /
                                             DriveInfo->DiskGeometry.BytesPerSector);

            CurrentTransferBytes = CurrentTransferSectors * DriveInfo->DiskGeometry.BytesPerSector;

            INFO_(FLOPPY, "ReadWritePassive: limiting transfer to 0x%x bytes (0x%x sectors) due to map registers\n",
                  CurrentTransferBytes, CurrentTransferSectors);
        }

        /* set up this round's dma operation */
        /* param 2 is ReadOperation --> opposite of WriteToDevice that IoMapTransfer takes.  BAD MS. */
        KeFlushIoBuffers(Irp->MdlAddress, !WriteToDevice, TRUE);

        IoMapTransfer(DriveInfo->ControllerInfo->AdapterObject, Irp->MdlAddress,
                      DriveInfo->ControllerInfo->MapRegisterBase,
                      (PVOID)((ULONG_PTR)MmGetMdlVirtualAddress(Irp->MdlAddress) + TransferByteOffset),
                      &CurrentTransferBytes, WriteToDevice);

        /*
         * Read or Write
         */
        KeClearEvent(&DriveInfo->ControllerInfo->SynchEvent);

        /* Issue the read/write command to the controller.  Note that it expects the opposite of WriteToDevice. */
        if(HwReadWriteData(DriveInfo->ControllerInfo, !WriteToDevice, DriveInfo->UnitNumber, Cylinder, Head, StartSector,
                           DriveInfo->BytesPerSectorCode, DriveInfo->DiskGeometry.SectorsPerTrack, Gap, 0xff) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "ReadWritePassive(): HwReadWriteData returned failure; unable to read; completing with STATUS_UNSUCCESSFUL\n");
            RWFreeAdapterChannel(DriveInfo->ControllerInfo->AdapterObject);
            IoCompleteRequest(Irp, IO_NO_INCREMENT);
            StopMotor(DriveInfo->ControllerInfo);
            return ;
        }

        INFO_(FLOPPY, "ReadWritePassive(): HwReadWriteData returned -- waiting on event\n");

        /*
         * At this point, we block and wait for an interrupt
         * FIXME: this seems to take too long
         */
        WaitForControllerInterrupt(DriveInfo->ControllerInfo);

        /* Read is complete; flush & free adapter channel */
        IoFlushAdapterBuffers(DriveInfo->ControllerInfo->AdapterObject, Irp->MdlAddress,
                              DriveInfo->ControllerInfo->MapRegisterBase,
                              (PVOID)((ULONG_PTR)MmGetMdlVirtualAddress(Irp->MdlAddress) + TransferByteOffset),
                              CurrentTransferBytes, WriteToDevice);

        /* Read the results from the drive */
        if(HwReadWriteResult(DriveInfo->ControllerInfo) != STATUS_SUCCESS)
        {
            WARN_(FLOPPY, "ReadWritePassive(): HwReadWriteResult returned failure; unable to read; completing with STATUS_UNSUCCESSFUL\n");
            HwDumpRegisters(DriveInfo->ControllerInfo);
            RWFreeAdapterChannel(DriveInfo->ControllerInfo->AdapterObject);
            IoCompleteRequest(Irp, IO_NO_INCREMENT);
            StopMotor(DriveInfo->ControllerInfo);
            return ;
        }

        TransferByteOffset += CurrentTransferBytes;
    }

    RWFreeAdapterChannel(DriveInfo->ControllerInfo->AdapterObject);

    /* That's all folks! */
    INFO_(FLOPPY, "ReadWritePassive(): success; Completing with STATUS_SUCCESS\n");
    Irp->IoStatus.Status = STATUS_SUCCESS;
    Irp->IoStatus.Information = Length;
    IoCompleteRequest(Irp, IO_DISK_INCREMENT);
    StopMotor(DriveInfo->ControllerInfo);
}