[PATCH 1/4] i915/gem: drop wbinvd_on_all_cpus usage

[Michael Cheng]

On 2022-03-21 3:30 a.m., Tvrtko Ursulin wrote:

>
> On 19/03/2022 19:42, Michael Cheng wrote:
>> Previous concern with using drm_clflush_sg was that we don't know 
>> what the
>> sg_table is pointing to, thus the usage of wbinvd_on_all_cpus to flush
>> everything at once to avoid paranoia.
>
> And now we know, or we know it is not a concern?
>
>> To make i915 more architecture-neutral and be less paranoid, lets 
>> attempt to
>
> "Lets attempt" as we don't know if this will work and/or what can/will 
> break?

Yes, but it seems like there's no regression with IGT .

If there's a big hit in performance, or if this solution gets accepted 
and the bug reports come flying in, we can explore other solutions. But 
speaking to Dan Vetter, ideal solution would be to avoid any calls 
directly to wbinvd, and use drm helpers in place.

+Daniel for any extra input.

>> use drm_clflush_sg to flush the pages for when the GPU wants to read
>> from main memory.
>>
>> Signed-off-by: Michael Cheng <michael.cheng at intel.com>
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 9 ++-------
>>   1 file changed, 2 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c 
>> b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> index f5062d0c6333..b0a5baaebc43 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> @@ -8,6 +8,7 @@
>>   #include <linux/highmem.h>
>>   #include <linux/dma-resv.h>
>>   #include <linux/module.h>
>> +#include <drm/drm_cache.h>
>>     #include <asm/smp.h>
>>   @@ -250,16 +251,10 @@ static int 
>> i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *obj)
>>        * DG1 is special here since it still snoops transactions even 
>> with
>>        * CACHE_NONE. This is not the case with other HAS_SNOOP 
>> platforms. We
>>        * might need to revisit this as we add new discrete platforms.
>> -     *
>> -     * XXX: Consider doing a vmap flush or something, where possible.
>> -     * Currently we just do a heavy handed wbinvd_on_all_cpus() here 
>> since
>> -     * the underlying sg_table might not even point to struct pages, 
>> so we
>> -     * can't just call drm_clflush_sg or similar, like we do 
>> elsewhere in
>> -     * the driver.
>>        */
>>       if (i915_gem_object_can_bypass_llc(obj) ||
>>           (!HAS_LLC(i915) && !IS_DG1(i915)))
>> -        wbinvd_on_all_cpus();
>> +        drm_clflush_sg(pages);
>
> And as noticed before, drm_clfush_sg still can call wbinvd_on_all_cpus 
> so are you just punting the issue somewhere else? How will it be 
> solved there?
>
Instead of calling an x86 asm directly, we are using what's available to 
use to make the driver more architecture neutral. Agreeing with Thomas, 
this solution falls within the "prefer range-aware clflush apis", and 
since some other generation platform doesn't support clflushopt, it will 
fall back to using wbinvd.
> Regards,
>
> Tvrtko
>
>>         sg_page_sizes = i915_sg_dma_sizes(pages->sgl);
>>       __i915_gem_object_set_pages(obj, pages, sg_page_sizes);