[PATCH] drm/radeon: avoid page fault during gpu reset

[Christian König]

Am 28.01.20 um 14:15 schrieb Andreas Messer:
> On Sat, Jan 25, 2020 at 07:01:36PM +0000, Koenig, Christian wrote:
>>
>> Am 25.01.2020 19:47 schrieb Andreas Messer <andi at bastelmap.de>:
>> When backing up a ring, validate pointer to avoid page fault.
>> [ cut description / kernel messages ]
>>
>> NAK, that was suggested multiple times now and is essentially the wrong
>> approach.
>>
>> The problem is that the value is invalid because the hardware is not
>> functional any more. Returning here without backing up the ring just
>> papers over the real problem.
>>
>> This is just the first occurance of this and you would need to fix a
>> couple of hundred register accesses (both inside and outside of the
>> driver) to make that really work reliable.
> Sure, it wont fix the hardware. But since the page fault is most prominent
> part in kernel log, people will continue suggesting it. With that change,
> the kernel messages are full of ring and atom bios timeouts and might make
> users more likely to consider a hardware issue in the first place.

That is correct, but the problem is that we currently have 2209 places 
where we read a register and usually expect that the values to be in a 
valid range.

If you really want to avoid all crashes you would need to audit and fix 
all occurrences where for example the register value is used as index in 
an array or similar.

And the radeon code is only the beginning, the whole PCIe subsystem 
would need an audit in a similar way. That is a huge lot of work we are 
not willing to do.

>   Anyway:
>
>> The only advice I can give you is to replace the hardware. From
>> experience those symptoms mean that your GPU will die rather soon.
> I think my hardware is fine. I have monitored gpu temp and fan pwm now for
> a while and found the pwm to be driven at ~60% only although the gpu
> already got quite high temperature during gameplay. When forcing the pwm
> to ~80% no crash occurs anymore. I suppose it is not the GPU crashing but
> instead the VRMs, not getting enough airflow.
>
> I have compared the Bios fan tables of my card with them of other cards
> bios (downloaded from web) of same GPU type and similar design.
> Although they differ in cooler construction and used fan, all of them
> despite one model have exactly the same fan regulation points with PWMHigh
> at 80% for 90°C. This single model with other settings has 100% for this
> temp and generally much more sane looking regulation curve.
>
> I suppose most of the vendors just copied some reference design,
> maybe the vendor's windows driver adjust the curve to a better one,
> I don't know.
>
> I think I'll add some sysfs attributes or module parameter to adjust
> the curve to my needs.

The issue is that this is most likely not a temperature problem at all. 
If you have a temperature problem the ASIC usually just hangs in a 
shader or so, but the BIF is still fully functional (e.g. you can probe 
PCI-IDs etc...).

That looks more like the ESD protection is kicking in for some reason. 
In other words what you got here is a cold/broken solder point on the 
SMD components which happens to loose contact because the material 
expands when it warms up.

That is a serious hardware fault and a really good indicator that you 
should replace the faulty component ASAP.

Regards,
Christian.

>
>> [ Patch cut out ]
> cheers,
> Andreas
>
>
>
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