Re: PROM vbios fetching issues

[Martin Peres <martin.peres-GANU6spQydw@public.gmane.org>]

On 25/03/2014 17:17, Christian Zander wrote:
> On Mon, Mar 24, 2014 at 11:59:46AM -0700, Martin Peres wrote:
>> Hello,
>>
>> One of my GPU (GK107/NVE7) fails to properly fetch its vbios from PROM
>> at boot time but, if I blacklist the module and load it myself later on,
>> it always succeeds. To make things weirder, the same card works great on
>> another computer.
>>
>> Here is the relevant code in Nouveau to fetch the vbios from PROM:
>> http://code.woboq.org/linux/linux/drivers/gpu/drm/nouveau/core/subdev/bios/base.c.html#nouveau_bios_shadow_prom
>>
>> The only solution I found to this problem is to try up to 16 times in a
>> row to fetch the vbios using PROM. If the 16 tries yield a bad signature
>> or checksum, I just give up and let it try other techniques. Here is the
>> patch[1].
>>
>> With this hack in place, the number of retries needed to be able to
>> correctly fetch the vbios ranges from 0 to 13. I see no consistency in
>> this. I tried reading the vbios 32 bits by 32 bits (instead of 8 by 8),
>> disabling local IRQs and generating more activity on the PCIe port
>> before fetching (and making sure the value read is always the same), all
>> solutions yielded no improvement.
>>
>> It could be a brownout problem but this would be the only manifestation
>> of this problem. Moreover, some cards are known to have problems with
>> PROM[2] so mine is definitely not the only one.
>>
>> I'm at a loss here, should we wait on anything before reading from PROM?
>> The mmiotrace I have doesn't seem to suggest we should :s I wonder if
>> you have had this issue before and what the nvkm does here.
>>
> Hi Martin,
>
> a functional regression in GK10x prevents your GK107 GPU
> from properly decoding 8-bit and 16-bit accesses to
> the PROM registers. While these unaligned accesses work on
> most GPUs, it is best not to rely on them. You can ensure
> reliable retrieval of the VBIOS via the PROM registers
> by using 32-bit register accesses across all GPUs.
>
> Thank you, Christian.

Hi Christian,

Thanks, that was what went wrong. I thought I had tested 32-bit accesses 
only but
Imust have forgotten to update the vbios size read...

Thanks again for this quick answer,

Martin