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

[Rob Clark]

On Thu, May 30, 2024 at 2:22 AM Bibek Kumar Patro
<quic_bibekkum@xxxxxxxxxxx> wrote:
>
>
>
> On 5/28/2024 9:38 PM, Rob Clark wrote:
> > On Tue, May 28, 2024 at 6:06 AM Dmitry Baryshkov
> > <dmitry.baryshkov@xxxxxxxxxx> wrote:
> >>
> >> On Tue, May 28, 2024 at 02:59:51PM +0200, 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).
> >>
> >> I'd second the request here. If another driver has to control the
> >> behaviour of another driver, please add corresponding API for that.
> >
> > We have adreno_smmu_priv for this purpose ;-)
> >
>
> Thanks Rob for pointing to this private interface structure between smmu
> and gpu. I think it's similar to what you're trying to implement here
> https://lore.kernel.org/all/CAF6AEGtm-KweFdMFvahH1pWmpOq7dW_p0Xe_13aHGWt0jSbg8w@xxxxxxxxxxxxxx/#t
> I can add an api "set_actlr_prr()" with smmu_domain cookie, page pointer
> as two parameters. This api then can be used by drm/msm driver to carry
> out the prr implementation by simply calling this.
> Would this be okay Rob,Konrad,Dmitry?
> Let me know if any other suggestions you have in mind as well regarding
> parameters and placement.

Hey Bibek, quick question.. is ACTLR preserved across a suspend/resume
cycle?  Or does it need to be reprogrammed on resume?  And same
question for these two PRR related regs:

  /* Global SMMU register offsets */
  #define KGSL_IOMMU_PRR_CFG_LADDR        0x6008
  #define KGSL_IOMMU_PRR_CFG_UADDR        0x600c

(ie. high/low 32b of the PRR page)

I was starting to type up a patch to add PRR configuration, but
depending on whether it interacts with suspend/resume, it might be
better form arm-smmu-qcom.c to just always enable and configure PRR
(including allocating a page to have an address to program into
PRR_CFG_LADDR/UADDR), and instead add an interface to return the PRR
page?  I think there is no harm in unconditionally configuring PRR for
gpu smmu.

BR,
-R

> Thanks & regards,
> Bibek
>
> > BR,
> > -R
> >
> >>>
> >>>>
> >>>>>> 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.
> >>>
> >>> Konrad
> >>
> >> --
> >> With best wishes
> >> Dmitry