Re: [PATCHv3 7/7] drm/msm/a6xx: Add support for using system cache(LLC)

[Rob Clark <robdclark@gmail.com>]

On Fri, Jul 3, 2020 at 7:53 AM Sai Prakash Ranjan
<saiprakash.ranjan@codeaurora.org> wrote:
>
> Hi Will,
>
> On 2020-07-03 19:07, Will Deacon wrote:
> > On Mon, Jun 29, 2020 at 09:22:50PM +0530, Sai Prakash Ranjan wrote:
> >> diff --git a/drivers/gpu/drm/msm/msm_iommu.c
> >> b/drivers/gpu/drm/msm/msm_iommu.c
> >> index f455c597f76d..bd1d58229cc2 100644
> >> --- a/drivers/gpu/drm/msm/msm_iommu.c
> >> +++ b/drivers/gpu/drm/msm/msm_iommu.c
> >> @@ -218,6 +218,9 @@ static int msm_iommu_map(struct msm_mmu *mmu,
> >> uint64_t iova,
> >>              iova |= GENMASK_ULL(63, 49);
> >>
> >>
> >> +    if (mmu->features & MMU_FEATURE_USE_SYSTEM_CACHE)
> >> +            prot |= IOMMU_SYS_CACHE_ONLY;
> >
> > Given that I think this is the only user of IOMMU_SYS_CACHE_ONLY, then
> > it
> > looks like it should actually be a property on the domain because we
> > never
> > need to configure it on a per-mapping basis within a domain, and
> > therefore
> > it shouldn't be exposed by the IOMMU API as a prot flag.
> >
> > Do you agree?
> >
>
> GPU being the only user is for now, but there are other clients which
> can use this.
> Plus how do we set the memory attributes if we do not expose this as
> prot flag?

It does appear that the downstream kgsl driver sets this for basically
all mappings.. well there is some conditional stuff around
DOMAIN_ATTR_USE_LLC_NWA but it seems based on the property of the
domain.  (Jordan may know more about what that is about.)  But looks
like there are a lot of different paths into iommu_map in kgsl so I
might have missed something.

Assuming there isn't some case where we specifically don't want to use
the system cache for some mapping, I think it could be a domain
attribute that sets an io_pgtable_cfg::quirks flag

BR,
-R