Re: [Qemu-devel] [PULL 48/49] i386: populate floppy drive information in DSDT

[Laszlo Ersek]

On 02/13/16 18:26, Kevin O'Connor wrote:
> On Tue, Feb 09, 2016 at 07:36:12PM +0100, Laszlo Ersek wrote:
>> On 02/09/16 17:22, John Snow wrote:
>>> On 02/09/2016 10:52 AM, Roman Kagan wrote:
>>>> On Mon, Feb 08, 2016 at 03:20:47PM -0500, John Snow wrote:
>>>>> On 02/08/2016 08:14 AM, Roman Kagan wrote:
>>>>>> On Fri, Feb 05, 2016 at 07:25:07PM +0100, Igor Mammedov wrote:
>>>>>>>> +    aml_append(fdi,
>>>>>>>> +        aml_int(cylinders - 1));  /* Maximum Cylinder Number */
>>>>>>> this puts uint64_t(-1) in AML i.e. cylinders == 0 and overflow happens 
>>>>>>> here
>>>>>>>
>>>>>>> CCing Jon
>>>>>>
>>>>>> I guess this is the effect of John's fdc rework.  I used to think zero
>>>>>> geometry was impossible at the time this patch was developed.
>>>>>>
>>>>>> I wonder if it hasn't been fixed already by
>>>>>>
>>>>>>   commit fd9bdbd3459e5b9d51534f0747049bc5b6145e07
>>>>>>   Author: John Snow <address@hidden>
>>>>>>   Date:   Wed Feb 3 11:28:55 2016 -0500
>>>>>>
>>>>>>       fdc: fix detection under Linux
>>>>>
>>>>> Yes, hopefully solved on my end. The geometry values for an empty disk
>>>>> are not well defined (they certainly don't have any *meaning*) so if you
>>>>> are populating tables based on an empty drive, I just hope you also have
>>>>> the mechanisms needed to update said tables when the media changes.
>>>>
>>>> I don't.  At the time the patch was developed there basically were no
>>>> mechanisms to update the geometry at all (and this was what you patchset
>>>> addressed, in particular, wasn't it?) so I didn't care.
>>>>
>>>
>>> That's not true.
>>>
>>> You could swap different 1.44MB-class diskettes for other geometries,
>>> check this out:
>>>
>>> static const FDFormat fd_formats[] = {
>>>     /* First entry is default format */
>>>     /* 1.44 MB 3"1/2 floppy disks */
>>>     { FDRIVE_DRV_144, 18, 80, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 20, 80, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 21, 80, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 21, 82, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 21, 83, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 22, 80, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 23, 80, 1, FDRIVE_RATE_500K, },
>>>     { FDRIVE_DRV_144, 24, 80, 1, FDRIVE_RATE_500K, },
>>> ...
>>>
>>> You absolutely could get different sector and track counts before my
>>> patchset.
>>>
>>>
>>>> Now if it actually has to be fully dynamic it's gonna be more
>>>> involved...
>>>>
>>>>> What do the guests use these values for? Are they fixed at boot?
>>>>
>>>> Only Windows guests use it so it's hard to tell.  I can only claim that
>>>> if I stick bogus values into that ACPI object the guest fails to read
>>>> the floppy.
>>
>> We discussed this with John a bit on IRC.
>>
>> In my opinion, the real mess in this case is in the ACPI spec itself. If
>> you re-read the _FDI control method's description, the Package that it
>> returns contains *dynamic* geometry data, about the *disk* (not *drive*):
>>
>> - Maximum Cylinder Number // Integer (WORD)
>> - Maximum Sector Number   // Integer (WORD)
>> - Maximum Head Number     // Integer (WORD)
> 
> FWIW, that's not how I read the ACPI specification.  I read it as
> saying that the information should be filled with the maximum number
> of CHS that the drive can support.  So, even if a smaller disk happens
> to be in the drive the maximum the drive supports would not change.

If that works with Windows, I think it would be optimal for QEMU.

Thanks!
Laszlo

> 
> Also, FWIW, SeaBIOS uses the standard 1.44MB floppy controller timing
> information even if a 5.25 drive is found - as far as I know this
> information is only ever used on PIO to the floppy controller and the
> QEMU floppy controller doesn't care what timing parameters it is
> programmed with.
> 
> -Kevin
>