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

Bibek Kumar Patro
2024-10-30T11:48:53Z

On 10/29/2024 6:17 PM, 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.

This seems to be a good idea to me . I am thinking of this approach
based on your suggestion,
i.e. we can bind the original workaround in
arm_mmu500_reset implementation within ARM_SMMU_WORKAROUND_BROKEN_CPRE
config (defualts to on, CPRE would be disabled) and in QCOM SoCs default
it to off
(when ARM_SMMU_QCOM=Y -> switch ARM_SMMU_WORKAROUND_BROKEN_CPRE=N).

In silicon-errata.rst would updating ARM_SMMU_WORKAROUND_BROKEN_CPRE be
okay , as the config names are based on erratum number.

Thanks & regards,
Bibek

>
> As an aside, I'm happy with the rest of the series now.
>
> Will