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[Qemu-devel] Regression with floppy drive controller

From: Philippe Mathieu-Daudé
Subject: [Qemu-devel] Regression with floppy drive controller
Date: Tue, 20 Aug 2019 12:25:52 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0

[cross posting QEMU & SeaBIOS]


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 <address@hidden>
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?



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

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