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

[Dr. David Alan Gilbert]

* Philippe Mathieu-Daudé ([e-mail]) wrote:
> On 8/20/19 3:38 PM, Philippe Mathieu-Daudé wrote:
> > 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 
> > > > 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.
>
> Looking at the fdc timer I noticed it use a static '50 ns' magic value.

That's not 50ns
> Increasing this value allows the floppy image to boot again, using this
> snippet:
>
> -- >8 --
> diff --git a/hw/block/fdc.c b/hw/block/fdc.c
> index 9b24cb9b85..5fc54073fd 100644
> --- a/hw/block/fdc.c
> +++ b/hw/block/fdc.c
> @@ -2134,7 +2134,7 @@ static void fdctrl_handle_readid(FDCtrl *fdctrl,
> int direction)
>  cur_drv->head = (fdctrl->fifo[1] >> 2) & 1;
>  timer_mod(fdctrl->result_timer, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) +
>         (NANOSECONDS_PER_SECOND / 50));

That's 1/50th of a second in ns.
>         (NANOSECONDS_PER_SECOND / 5000));

I'm not too sure about readid; but assuming we're rotating at 360rpm,
that's 6 revolutions/second, and 18 sectors/track = 108 sectors/second
(half of that for a double density disk).

So, the wait for a sector to spin around and read feels like it should
be in the region of 1/108 of a second + some latency - so 1/50th of a
second would seem to be in the ballpark or being right, where as 1/5000
of a second is way too fast for a poor old floppy.

Dave
> }
> static void fdctrl_handle_format_track(FDCtrl *fdctrl, int direction)
>
> Any idea what is the correct value to use here?
>
> Regards,
>
> Phil.
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