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

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Message-ID: <omswcicgc2kqd6gp4bebd43sklfs2wqyaorhfyb2wumoeo6v74@gaay3p5m46xi>
Date: Tue, 28 May 2024 16:06:45 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Konrad Dybcio <konrad.dybcio@...aro.org>
Cc: Bibek Kumar Patro <quic_bibekkum@...cinc.com>, 
	Rob Clark <robdclark@...il.com>, will@...nel.org, robin.murphy@....com, joro@...tes.org, 
	jsnitsel@...hat.com, quic_bjorande@...cinc.com, mani@...nel.org, 
	quic_eberman@...cinc.com, robdclark@...omium.org, u.kleine-koenig@...gutronix.de, 
	robh@...nel.org, vladimir.oltean@....com, quic_pkondeti@...cinc.com, 
	quic_molvera@...cinc.com, linux-arm-msm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 3/5] iommu/arm-smmu: introduction of ACTLR for custom
 prefetcher settings

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@...cinc.com> 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@...cinc.com>
> > > > > > ---
> > > 
> > > [...]
> > > 
> > > > > > +
> > > > > > +               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.

> 
> > 
> > > > 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