Re: [PATCH v16 1/5] iommu/arm-smmu: re-enable context caching in smmu reset operation

Dmitry Baryshkov
2024-11-06T07:44:46Z

On Tue, Nov 05, 2024 at 11:37:24AM +0000, Will Deacon wrote:
> On Mon, Nov 04, 2024 at 01:10:12PM +0200, Dmitry Baryshkov wrote:
> > On Tue, Oct 29, 2024 at 12:47:09PM +0000, Will Deacon wrote:
> > > On Fri, Oct 25, 2024 at 07:51:22PM +0530, Bibek Kumar Patro wrote:
> > > >
> > > >
> > > > On 10/24/2024 6:22 PM, Will Deacon wrote:
> > > > > On Tue, Oct 08, 2024 at 06:24:06PM +0530, Bibek Kumar Patro wrote:
> > > > > > Default MMU-500 reset operation disables context caching in
> > > > > > prefetch buffer. It is however expected for context banks using
> > > > > > the ACTLR register to retain their prefetch value during reset
> > > > > > and runtime suspend.
> > > > > >
> > > > > > Replace default MMU-500 reset operation with Qualcomm specific reset
> > > > > > operation which envelope the default reset operation and re-enables
> > > > > > context caching in prefetch buffer for Qualcomm SoCs.
> > > > > >
> > > > > > Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
> > > > > > Signed-off-by: Bibek Kumar Patro <quic_bibekkum@quicinc.com>
> > > > > > ---
> > > > > > drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 45 ++++++++++++++++++++--
> > > > > > 1 file changed, 42 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > > > > > index 087fb4f6f4d3..0cb10b354802 100644
> > > > > > --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > > > > > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > > > > > @@ -16,6 +16,16 @@
> > > > > >
> > > > > > #define QCOM_DUMMY_VAL -1
> > > > > >
> > > > > > +/*
> > > > > > + * SMMU-500 TRM defines BIT(0) as CMTLB (Enable context caching in the
> > > > > > + * macro TLB) and BIT(1) as CPRE (Enable context caching in the prefetch
> > > > > > + * buffer). The remaining bits are implementation defined and vary across
> > > > > > + * SoCs.
> > > > > > + */
> > > > > > +
> > > > > > +#define CPRE (1 << 1)
> > > > > > +#define CMTLB (1 << 0)
> > > > > > +
> > > > > > static struct qcom_smmu *to_qcom_smmu(struct arm_smmu_device *smmu)
> > > > > > {
> > > > > > return container_of(smmu, struct qcom_smmu, smmu);
> > > > > > @@ -396,11 +406,40 @@ static int qcom_smmu_def_domain_type(struct device *dev)
> > > > > > return match ? IOMMU_DOMAIN_IDENTITY : 0;
> > > > > > }
> > > > > >
> > > > > > +static int qcom_smmu500_reset(struct arm_smmu_device *smmu)
> > > > > > +{
> > > > > > + int ret;
> > > > > > + u32 val;
> > > > > > + int i;
> > > > > > +
> > > > > > + ret = arm_mmu500_reset(smmu);
> > > > > > + if (ret)
> > > > > > + return ret;
> > > > > > +
> > > > > > + /*
> > > > > > + * arm_mmu500_reset() disables CPRE which is re-enabled here.
> > > > > > + * The errata for MMU-500 before the r2p2 revision requires CPRE to be
> > > > > > + * disabled. The arm_mmu500_reset function disables CPRE to accommodate all
> > > > > > + * RTL revisions. Since all Qualcomm SoCs are on the r2p4 revision, where
> > > > > > + * the CPRE bit can be enabled, the qcom_smmu500_reset function re-enables
> > > > > > + * the CPRE bit for the next-page prefetcher to retain the prefetch value
> > > > > > + * during reset and runtime suspend operations.
> > > > > > + */
> > > > > > +
> > > > > > + for (i = 0; i < smmu->num_context_banks; ++i) {
> > > > > > + val = arm_smmu_cb_read(smmu, i, ARM_SMMU_CB_ACTLR);
> > > > > > + val |= CPRE;
> > > > > > + arm_smmu_cb_write(smmu, i, ARM_SMMU_CB_ACTLR, val);
> > > > > > + }
> > > > >
> > > > > If CPRE only needs to be disabled prior to r2p2, then please teach the
> > > > > MMU-500 code about that instead of adding qualcomm-specific logic here.
> > > > >
> > > >
> > > > Doing this on MMU-500 code would make it generic and reflect for SoC of all
> > > > the vendors on this platform.
> > > > We can make sure that it won't cause any problems in Qualcomm SoCs as we
> > > > have been enabling this since for some years now and could not
> > > > observe/reproduce any issues around these errata.
> > >
> > > Unless you can explain definitively hy that's the case, I still don't
> > > think we should be second-guessing the core SMMU driver code in the
> > > Qualcomm backend.
> > >
> > > > But we won't be able to guarantee the same behavior in SoC for other vendors
> > > > where these errata might still be applicable as per [1] and [2].
> > > > So as per my understanding it's safe to include in Qualcomm specific
> > > > implementation and not changing the default behavior in all other vendors'
> > > > SoC even if they are not prior to r2p2 revision [3].
> > >
> > > If you want to gate the errata workarounds on policy, then please follow
> > > what we do for the CPU: add a Kconfig option (e.g.
> > > ARM_SMMU_WORKAROUND_BROKEN_CPRE) which defaults to "on" (assuming that
> > > the relevant errata aren't all "rare") and update silicon-errata.rst
> > > accordingly.
> > >
> > > Then you can choose to disable them in your .config if you're happy to
> > > pick up the pieces.
> >
> > Is it actually going to work? For most of the CPU errata we can detect
> > and limit the workarounds to some class of CPUs. For SMMU, if I'm not
> > misunderstanding something, the errata will be enabled by default for
> > all SMMU-500 implementation, so only very few kernels, targeting only
> > the Qualcomm hardware, can get that disabled.
>
> We can add checks based on rXpY per the erratum documentation, but Robin
> was saying elsewhere in the thread that some of them are still open (i.e.
> unfixed).
>
> So ultimately, the decision to disable workarounds for known errata on
> broken hardware is going to be a niche sport, yes.

Yes, I understand that. I'm just trying to understand if we can have a
better solution than having errata workarounds enabled on a majority of
the kernels (which means less testing for the non-workaround-enabled
setup on Qualcomm devices).

--
With best wishes
Dmitry