linux-kernel - Re: [PATCH v14 1/6] iommu/arm-smmu: re-enable context caching in smmu reset operation

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Message-ID: <fe5fcd06-ff28-4171-aa22-1bdc1e8510d0@arm.com>
Date: Fri, 30 Aug 2024 13:53:57 +0100
From: Robin Murphy <robin.murphy@....com>
To: Bibek Kumar Patro <quic_bibekkum@...cinc.com>, robdclark@...il.com,
 will@...nel.org, joro@...tes.org, jgg@...pe.ca, jsnitsel@...hat.com,
 robh@...nel.org, krzysztof.kozlowski@...aro.org, quic_c_gdjako@...cinc.com,
 dmitry.baryshkov@...aro.org, konrad.dybcio@...aro.org
Cc: iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v14 1/6] iommu/arm-smmu: re-enable context caching in smmu
 reset operation

On 16/08/2024 6:42 pm, 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@...aro.org>
> Signed-off-by: Bibek Kumar Patro <quic_bibekkum@...cinc.com>
> ---
>   drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 36 ++++++++++++++++++++--
>   1 file changed, 33 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 36c6b36ad4ff..8ac1850b852f 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);
> @@ -381,11 +391,31 @@ 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 */

I still think it would be good to document why we think this is OK, 
given the reasons for disabling CPRE to begin with.

Thanks,
Robin.

> +	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);
> +	}
> +
> +	return 0;
> +}
> +
>   static int qcom_sdm845_smmu500_reset(struct arm_smmu_device *smmu)
>   {
>   	int ret;
> 
> -	arm_mmu500_reset(smmu);
> +	qcom_smmu500_reset(smmu);
> 
>   	/*
>   	 * To address performance degradation in non-real time clients,
> @@ -412,7 +442,7 @@ static const struct arm_smmu_impl qcom_smmu_500_impl = {
>   	.init_context = qcom_smmu_init_context,
>   	.cfg_probe = qcom_smmu_cfg_probe,
>   	.def_domain_type = qcom_smmu_def_domain_type,
> -	.reset = arm_mmu500_reset,
> +	.reset = qcom_smmu500_reset,
>   	.write_s2cr = qcom_smmu_write_s2cr,
>   	.tlb_sync = qcom_smmu_tlb_sync,
>   #ifdef CONFIG_ARM_SMMU_QCOM_DEBUG
> @@ -445,7 +475,7 @@ static const struct arm_smmu_impl qcom_adreno_smmu_v2_impl = {
>   static const struct arm_smmu_impl qcom_adreno_smmu_500_impl = {
>   	.init_context = qcom_adreno_smmu_init_context,
>   	.def_domain_type = qcom_smmu_def_domain_type,
> -	.reset = arm_mmu500_reset,
> +	.reset = qcom_smmu500_reset,
>   	.alloc_context_bank = qcom_adreno_smmu_alloc_context_bank,
>   	.write_sctlr = qcom_adreno_smmu_write_sctlr,
>   	.tlb_sync = qcom_smmu_tlb_sync,
> --
> 2.34.1
>