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

[Michael Cheng]

On 2022-03-21 4:07 a.m., Thomas Hellström wrote:
>
> On 3/21/22 11:30, 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?
>>
>>> 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?
>
> I think in this case, drm_clflush_sg() can't be immediately used, 
> because pages may not contain actual page pointers; might be just the 
> dma address. It needs to be preceded with a dmabuf vmap.

Could you elaborate more with using a dmabuf vmap?

Doing a quick grep on drm_clflush_sg, were you thinking about something 
similar to the following?

if (obj->cache_dirty) {
WARN_ON_ONCE(IS_DGFX(i915));
obj->write_domain = 0;
if (i915_gem_object_has_struct_page(obj))
drm_clflush_sg(pages);
obj->cache_dirty = false;
}


Thanks,

Michael Cheng

> But otherwise this change, I figure, falls into the "prefer 
> range-aware apis" category; If the CPU supports it, flush the range 
> only, otherwise fall back to wbinvd().
>
> /Thomas
>
>
>>
>> Regards,
>>
>> Tvrtko
>>
>>>         sg_page_sizes = i915_sg_dma_sizes(pages->sgl);
>>>       __i915_gem_object_set_pages(obj, pages, sg_page_sizes);
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