Re: [Qemu-devel] Regression with floppy drive controller

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

On 8/20/19 3:12 PM, John Snow wrote:
> On 8/20/19 6:25 AM, Philippe Mathieu-Daudé wrote:
>> [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?
[...]
> 
> Well, that's unfortunate.
> 
> What version of QEMU shipped the SeaBIOS that caused the regression?

See https://bugs.launchpad.net/qemu/+bug/1840719/comments/3

QEMU commit 0b8f74488e, slighly before QEMU v3.1.0
(previous tag is v3.0.0).

But you can use v4.1.0 too, simply change the SeaBIOS bios.bin, i.e.:

  qemu$ git checkout v4.1.0

  qemu$ (cd roms/seabios && git checkout 4a6dbcea3e4~) && \
        make -C roms bios

Now pc-bios/bios.bin is built using the last commit working,

  qemu$ (cd roms/seabios && git checkout 4a6dbcea3e4) && \
        make -C roms bios

And you can reproduce the error.

Regards,

Phil.