[Qemu-devel] Regression with floppy drive controller

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

[cross posting QEMU & SeaBIOS]

Hello,

I'v been looking at a QEMU bug report [1] which bisection resulted in a
SeaBIOS commit:

4a6dbcea3e412fe12effa2f812f50dd7eae90955 is the first bad commit
commit 4a6dbcea3e412fe12effa2f812f50dd7eae90955
Author: Nikolay Nikolov <nickysn@users.sourceforge.net>
Date:   Sun Feb 4 17:27:01 2018 +0200

    floppy: Use timer_check() in floppy_wait_irq()

    Use timer_check() instead of using floppy_motor_counter in BDA for the
    timeout check in floppy_wait_irq().

    The problem with using floppy_motor_counter was that, after it reaches
    0, it immediately stops the floppy motors, which is not what is
    supposed to happen on real hardware. Instead, after a timeout (like in
    the end of every floppy operation, regardless of the result - success,
    timeout or error), the floppy motors must be kept spinning for
    additional 2 seconds (the FLOPPY_MOTOR_TICKS). So, now the
    floppy_motor_counter is initialized to 255 (the max value) in the
    beginning of the floppy operation. For IRQ timeouts, a different
    timeout is used, specified by the new FLOPPY_IRQ_TIMEOUT constant
    (currently set to 5 seconds - a fairly conservative value, but should
    work reliably on most floppies).

    After the floppy operation, floppy_drive_pio() resets the
    floppy_motor_counter to 2 seconds (FLOPPY_MOTOR_TICKS).

    This is also consistent with what other PC BIOSes do.


This commit improve behavior with real hardware, so maybe QEMU is not
modelling something or modelling it incorrectly?


Regards,

Phil.


PD: How to reproduce:

- Download Windows 98 SE floppy image from [2]

- Run QEMU using the 'isapc' machine:

  $ qemu-system-i386 -M isapc \
     -fda Windows\ 98\ Second\ Edition\ Boot.img

  SeaBIOS (version rel-1.11.0-11-g4a6dbce-prebuilt.qemu.org)
  Booting from Floppy...
  Boot failed: could not read the boot disk

[1] https://bugs.launchpad.net/qemu/+bug/1840719
[2] https://winworldpc.com/download/417d71c2-ae18-c39a-11c3-a4e284a2c3a5