Re: [PATCH 2/4] drm/xe: store bind time pat index to xe_bo

[Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>]

Hi Matthew, thanks for looking into these. Below few thoughts.

On 19.1.2024 17.45, Matthew Auld wrote:
> On 18/01/2024 15:27, Juha-Pekka Heikkila wrote:
>> Store pat index from xe_vma to xe_bo
>>
>> Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
>> ---
>>   drivers/gpu/drm/xe/xe_pt.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/xe/xe_pt.c b/drivers/gpu/drm/xe/xe_pt.c
>> index de1030a47588..4b76db698878 100644
>> --- a/drivers/gpu/drm/xe/xe_pt.c
>> +++ b/drivers/gpu/drm/xe/xe_pt.c
>> @@ -1252,6 +1252,10 @@ __xe_pt_bind_vma(struct xe_tile *tile, struct 
>> xe_vma *vma, struct xe_exec_queue
>>           return ERR_PTR(-ENOMEM);
>>       }
>> +    if (xe_vma_bo(vma)) {
>> +        xe_vma_bo(vma)->pat_index = vma->pat_index;
> 
> Multiple mappings will trash this I think. Is that OK for your usecase? 
> It can be useful to map the same resource as compressed and uncompressed 
> to facilitate in-place decompression/compression.

On i915 I think we did map framebuffers only once and did stay with it 
until fb was destroyed. XE_BO_SCANOUT_BIT is for buffers that are meant 
to be framebuffers? I could make it so pat index given first is not 
allowed to change for buffers with this bit set?

> 
> Also would be good to be clear about what happens if the KMD doesn't do 
> anything to prevent compression with non-tile4? Is it just a bit of 
> display corruption or something much worse that we need to prevent? Is 
> this just a best effort check to help userspace? Otherwise it is hard to 
> evaluate how solid we need to be here in our checking to prevent this 
> scenario. For example how is binding vs display races handled? What 
> happens if the bind appears after the display check?

For what happen with incorrect buffers going for display I've seen they 
are corrupted on screen but my testing is very minimal. On bspec 67158 
it just said linear and tile X formats are not supported with 
decompression on display, so it is broken config. Couldn't say generally 
how robust display hw is for broken configs. I remember Ville had found 
with TGL broken configs caused unrecoverable issues which followed ccs 
getting blocked on some steppings because it was only way to block 
broken config Ville found. I'll add Ville here on cc if he has views on 
this what's needed here for Xe2.

/Juha-Pekka

> 
>> +    }
>> +
>>       fence = xe_migrate_update_pgtables(tile->migrate,
>>                          vm, xe_vma_bo(vma), q,
>>                          entries, num_entries,