Re: [Intel-gfx] [PATCH] drm/i915/mtl: Increase guard pages when vt-d is enabled

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 02/11/2023 16:58, Andrzej Hajda wrote:
> On 02.11.2023 17:06, Radhakrishna Sripada wrote:
>> Experiments were conducted with different multipliers to VTD_GUARD macro
>> with multiplier of 185 we were observing occasional pipe faults when
>> running kms_cursor_legacy --run-subtest single-bo
>>
>> There could possibly be an underlying issue that is being 
>> investigated, for
>> now bump the guard pages for MTL.
>>
>> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2017
>> Cc: Gustavo Sousa <gustavo.sousa@intel.com>
>> Cc: Chris Wilson <chris.p.wilson@linux.intel.com>
>> Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_domain.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_domain.c 
>> b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
>> index 3770828f2eaf..b65f84c6bb3f 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_domain.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
>> @@ -456,6 +456,9 @@ i915_gem_object_pin_to_display_plane(struct 
>> drm_i915_gem_object *obj,
>>       if (intel_scanout_needs_vtd_wa(i915)) {
>>           unsigned int guard = VTD_GUARD;
>> +        if (IS_METEORLAKE(i915))
>> +            guard *= 200;
>> +
> 
> 200 * VTD_GUARD = 200 * 168 * 4K = 131MB
> 
> Looks insanely high, 131MB for padding, if this is before and after it 
> becomes even 262MB of wasted address per plane. Just signalling, I do 
> not know if this actually hurts.

Yeah this feels crazy. There must be some other explanation which is 
getting hidden by the crazy amount of padding so I'd rather we figured 
it out.

With 262MiB per fb how many fit in GGTT before eviction hits? N screens 
with double/triple buffering?

Regards,

Tvrtko

P.S. Where did the 185 from the commit message come from?