PROM vbios fetching issues

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

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.

Thanks,
Martin

[1] http://lists.freedesktop.org/archives/nouveau/2014-March/016590.html
[2] 
http://code.woboq.org/linux/linux/drivers/gpu/drm/nouveau/core/subdev/bios/base.c.html#144