[Intel-gfx] [PATCH] drm/i915: Suppress switch_mm emission between the same aliasing_ppgtt

[Martin Peres]

On 11/01/17 14:58, Joonas Lahtinen wrote:
> On ke, 2017-01-11 at 12:14 +0000, Chris Wilson wrote:
>> When switching between contexts using the aliasing_ppgtt, the VM is
>> shared. We don't need to reload the PD registers unless they are dirty.
>>
>> Martin Peres reported an issue that looks like corruption between
>> Haswell context switches, bisecting to commit f9326be5f1d3 ("drm/i915:
>> Rearrange switch_context to load the aliasing ppgtt on first use").
>> Switching between the same mm (the aliasing_ppgtt is used for all
>> contexts in this case) should be a nop, but appears to trigger some
>> side-effects in the context switch. However, as we know the switch
>> is redundant in this case, we can skip it and continue to ignore the
>> issue until somebody feels strong enough to investigate full-ppgtt on
>> gen7 again!
>>
>> Fixes: f9326be5f1d3 ("drm/i915: Rearrange switch_context to load the aliasing ppgtt on first use")
>> Reported-by: Martin Peres <martin.peres at linux.intel.com>
>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>> Cc: Martin Peres <martin.peres at linux.intel.com>
>
> Code looks good, could use the T-b's to verify.
>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
>
> Regards, Joonas
>

https://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=for-mupuf&id=cfe8f1043b45896af23e4a978020fe20e90c5072 
was actually the commit that massively improved the corruption I was 
seeing in one benchmark while this patch had no visible impact.

However, my problem was that i915.enable_ppgtt=2 was set in 
/etc/modprobe.d/... and I had completely forgotten about it.

So yeah, now you know that f9326be5f1d3 massively broke enable_ppgtt=2, 
but not sure what you want to do about it.

There is no hurry though, as the defaults are sane.

Sorry for the noise everyone, I hope that my painful manual bisects will 
be useful if someone wants to make the second mode work :)

Martin