Re: [PATCH v9 3/5] iommu/arm-smmu: introduction of ACTLR for custom prefetcher settings

[Bibek Kumar Patro]

On 5/28/2024 6:29 PM, Konrad Dybcio wrote:
> On 5/15/24 15:59, Bibek Kumar Patro wrote:
> > On 5/10/2024 6:32 PM, Konrad Dybcio wrote:
> > > On 10.05.2024 2:52 PM, Bibek Kumar Patro wrote:
> > > > On 5/1/2024 12:30 AM, Rob Clark wrote:
> > > > > On Tue, Jan 23, 2024 at 7:00 AM Bibek Kumar Patro
> > > > > <quic_bibekkum@xxxxxxxxxxx> wrote:
> > > > > > Currently in Qualcomm  SoCs the default prefetch is set to 1 which 
> > > > > > allows
> > > > > > the TLB to fetch just the next page table. MMU-500 features ACTLR
> > > > > > register which is implementation defined and is used for Qualcomm 
> > > > > > SoCs
> > > > > > to have a custom prefetch setting enabling TLB to prefetch the 
> > > > > > next set
> > > > > > of page tables accordingly allowing for faster translations.
> > > > > >
> > > > > > ACTLR value is unique for each SMR (Stream matching register) and 
> > > > > > stored
> > > > > > in a pre-populated table. This value is set to the register during
> > > > > > context bank initialisation.
> > > > > >
> > > > > > Signed-off-by: Bibek Kumar Patro <quic_bibekkum@xxxxxxxxxxx>
> > > > > > ---
> > >
> > > [...]
> > >
> > > > > > +
> > > > > > +               for_each_cfg_sme(cfg, fwspec, j, idx) {
> > > > > > +                       smr = &smmu->smrs[idx];
> > > > > > +                       if (smr_is_subset(smr, id, mask)) {
> > > > > > +                               arm_smmu_cb_write(smmu, cbndx, 
> > > > > > ARM_SMMU_CB_ACTLR,
> > > > > > +                                               actlrcfg[i].actlr);
> > > > >
> > > > > So, this makes ACTLR look like kind of a FIFO.  But I'm looking at
> > > > > downstream kgsl's PRR thing (which we'll need to implement vulkan
> > > > > sparse residency), and it appears to be wanting to set BIT(5) in ACTLR
> > > > > to enable PRR.
> > > > >
> > > > >           val = KGSL_IOMMU_GET_CTX_REG(ctx, KGSL_IOMMU_CTX_ACTLR);
> > > > >           val |= FIELD_PREP(KGSL_IOMMU_ACTLR_PRR_ENABLE, 1);
> > > > >           KGSL_IOMMU_SET_CTX_REG(ctx, KGSL_IOMMU_CTX_ACTLR, val);
> > > > >
> > > > > Any idea how this works?  And does it need to be done before or after
> > > > > the ACTLR programming done in this patch?
> > > > >
> > > > > BR,
> > > > > -R
> > > >
> > > > Hi Rob,
> > > >
> > > > Can you please help provide some more clarification on the FIFO 
> > > > part? By FIFO are you referring to the storing of ACTLR data in the 
> > > > table?
> > > >
> > > > Thanks for pointing to the downstream implementation of kgsl driver for
> > > > the PRR bit. Since kgsl driver is already handling this PRR bit's
> > > > setting, this makes setting the PRR BIT(5) by SMMU driver redundant.
> > >
> > > The kgsl driver is not present upstream.
> >
> > Right kgsl is not present upstream, it would be better to avoid 
> > configuring the PRR bit and can be handled by kgsl directly in 
> > downstream.
>
> No! Upstream is not a dumping ground to reduce your technical debt.
>
> There is no kgsl driver upstream, so this ought to be handled here, in
> the iommu driver (as poking at hardware A from driver B is usually not good
> practice).

Okay, so I see this point now. Driver B need to use hardware A's driver
exposed interface only to interact with the hardware functionality
instead of directly poking it. Agree on this, it looks to be an
appropriate approach.

> > > > Thanks for bringing up this point.
> > > > I will send v10 patch series removing this BIT(5) setting from the 
> > > > ACTLR
> > > > table.
> > >
> > > I think it's generally saner to configure the SMMU from the SMMU 
> > > driver..
> >
> > Yes, agree on this. But since PRR bit is not directly related to SMMU
> > configuration so I think it would be better to remove this PRR bit
> > setting from SMMU driver based on my understanding.
>
> Why is it not related? We still don't know what it does.

By not related, I meant to say this bit is used for GFX implementation 
instead of direct SMMU related configuration.

Thanks & regards,
Bibek

> Konrad