Re: [PATCH v7 2/2] drm/i915: use pat_index instead of cache_level

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On 09/05/2023 18:12, Yang, Fei wrote:
 > On 09/05/2023 00:48, fei.yang@xxxxxxxxx wrote:
 >> From: Fei Yang <fei.yang@xxxxxxxxx>
 >>
>> Currently the KMD is using enum i915_cache_level to set caching policy for >> buffer objects. This is flaky because the PAT index which really controls >> the caching behavior in PTE has far more levels than what's defined in the >> enum. In addition, the PAT index is platform dependent, having to translate >> between i915_cache_level and PAT index is not reliable, and makes the code
 >> more complicated.
 >>
>> From UMD's perspective there is also a necessity to set caching policy for >> performance fine tuning. It's much easier for the UMD to directly use PAT >> index because the behavior of each PAT index is clearly defined in Bspec. >> Having the abstracted i915_cache_level sitting in between would only cause >> more ambiguity. PAT is expected to work much like MOCS already works today,
 >> and by design userspace is expected to select the index that exactly
 >> matches the desired behavior described in the hardware specification.
 >>
>> For these reasons this patch replaces i915_cache_level with PAT index. Also >> note, the cache_level is not completely removed yet, because the KMD still >> has the need of creating buffer objects with simple cache settings such as >> cached, uncached, or writethrough. For kernel objects, cache_level is used >> for simplicity and backward compatibility. For Pre-gen12 platforms PAT can >> have 1:1 mapping to i915_cache_level, so these two are interchangeable. see
 >> the use of LEGACY_CACHELEVEL.
 >>
>> One consequence of this change is that gen8_pte_encode is no longer working >> for gen12 platforms due to the fact that gen12 platforms has different PAT >> definitions. In the meantime the mtl_pte_encode introduced specfically for
 >> MTL becomes generic for all gen12 platforms. This patch renames the MTL
>> PTE encode function into gen12_pte_encode and apply it to all gen12. Even >> though this change looks unrelated, but separating them would temporarily
 >> break gen12 PTE encoding, thus squash them in one patch.
 >>
>> Special note: this patch changes the way caching behavior is controlled in >> the sense that some objects are left to be managed by userspace. For such
 >> objects we need to be careful not to change the userspace settings.There
>> are kerneldoc and comments added around obj->cache_coherent, cache_dirty,
 >> and how to bypass the checkings by i915_gem_object_has_cache_level. For
 >> full understanding, these changes need to be looked at together with the
>> two follow-up patches, one disables the {set|get}_caching ioctl's and the
 >> other adds set_pat extension to the GEM_CREATE uAPI.
 >>
 >> Bspec: 63019
 >>
 >> Cc: Chris Wilson <chris.p.wilson@xxxxxxxxxxxxxxx>
 >> Signed-off-by: Fei Yang <fei.yang@xxxxxxxxx>
 >> Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
 >> Reviewed-by: Matt Roper <matthew.d.roper@xxxxxxxxx>

[snip]

 >> +                                          node.start,
 >> +                                          i915_gem_get_pat_index(i915,
>> + I915_CACHE_NONE), 0); >>                        wmb(); /* flush modifications to the GGTT (insert_page) */
 >>                } else {
 >>                        page_base += offset & PAGE_MASK;
>> @@ -1142,6 +1148,19 @@ int i915_gem_init(struct drm_i915_private *dev_priv)
 >>        unsigned int i;
 >>        int ret;
 >>
 >> +     /*
>> +      * In the proccess of replacing cache_level with pat_index a tricky >> +      * dependency is created on the definition of the enum i915_cache_level.
 >> +      * in case this enum is changed, PTE encode would be broken.
 >
 >_I_n

Sorry, what does this mean?

Start of sentence, capital 'i'.

[snip]

 > With a pinky promise to improve this all in the near future I won't
 > grumble to loudly. :) I haven't read all the details, I leave that to
 > other reviewers, and also assuming some final tweaks as indicated above
 > please.

Thanks for all the suggestions, really appreciated.
May I add your Acked-by?

I can't make myself do it since I really don't like the design that much. That's why I said I will not grumble too loudly.

Jira for follow up clean since we both agreed something more elegant is possible would be appreciated though.

Regards,

Tvrtko



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