[PATCH v17 1/1] drm/i915: Allow user to set cache at BO creation

[Zhang, Carl]

It is impossible to pass the PAT setting when sharing the fd.

  1.  All api , include vaapi, vulkan, gl... need such extension , the impact is huge.  Even these API level accept , some middle ware , such as ffmpeg... also will reject it.
  2.  PAT is intel specific implementation , and MTL , LNL has different value and meanings, you could not expect application developer understand the details, it should be transparent to application developer.
Thanks
Carl

From: Yang, Fei <fei.yang at intel.com>
Sent: Friday, June 9, 2023 11:13 PM
To: Andi Shyti <andi.shyti at linux.intel.com>; Zhang, Carl <carl.zhang at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>; Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>; Chris Wilson <chris at chris-wilson.co.uk>; Roper, Matthew D <matthew.d.roper at intel.com>; Justen, Jordan L <jordan.l.justen at intel.com>; Gu, Lihao <lihao.gu at intel.com>; Intel GFX <intel-gfx at lists.freedesktop.org>; DRI Devel <dri-devel at lists.freedesktop.org>
Subject: Re: [PATCH v17 1/1] drm/i915: Allow user to set cache at BO creation

> Hi Carl,
>
>>>> besides this, ask a dumb question.
>>>> How we retrieve the pat_index from a shared resource though dma_buf fd?
>>>> maybe we need to know whether it could be CPU cached if we want map it.
>>>> Of course, looks there are no real usage to access it though CPU.
>>>> Just use it directly without any pat related options ?
>>>
>>> I am not understanding. Do you want to ask the PAT table to the driver? Are
>>> you referring to the CPU PAT index?
>>>
>>> In any case, if I understood correctly, you don't necessarily always need to
>>> set the PAT options and the cache options will fall into the default values.
>>>
>>> Please let me know if I haven't answered the question.
>>>
>>
>> If mesa create a resource , then use DRM_IOCTL_PRIME_HANDLE_TO_FD convert it to a dma fd.
>> Then share it to media, media use DRM_IOCTL_PRIME_FD_TO_HANDLE convert it to a gem bo.
>> But media does not know the PAT index , because mesa create it and set it.
>> So, if media want to call DRM_IOCTL_I915_GEM_MMAP_OFFSET, media does not know whether it could be WB.
>
> That's a good point. To be honest I am not really sure how this
> is handled.
>
> Fei, Jordan? Do you have suggestion here?

Is it possible to pass the PAT setting when sharing the fd?
Or perhaps we should have kept the get_caching ioctl functional?
Joonas, could you chime in here?

> Andi
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