Re: [PATCH v3 4/8] iommu/arm-smmu-qcom: Emulate bypass by using context banks

[Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>]

On 2020-09-04 21:25, Bjorn Andersson wrote:
> Some firmware found on various Qualcomm platforms traps writes to S2CR
> of type BYPASS and writes FAULT into the register. In particular, this
> prevents us from marking the streams for the display controller as
> BYPASS to allow continued scanout of the screen through the
> initialization of the ARM SMMU.
> 
> This adds a Qualcomm specific cfg_probe function, which probes for the
> broken behavior of the S2CR registers and implements a custom
> alloc_context_bank() that when necessary allocates a context bank
> (without translation) for these domains as well.
> 
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
> 
> Changes since v2:
> - Move quirk from arm_smmudevice to qcom_smmu, as we localize the quirk
>   handling to the Qualcomm specific implemntation.
> 
>  drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 52 ++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> index 229fc8ff8cea..284761a1cd8e 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> @@ -11,8 +11,14 @@
> 
>  struct qcom_smmu {
>  	struct arm_smmu_device smmu;
> +	bool bypass_broken;
>  };
> 
> +static struct qcom_smmu *to_qcom_smmu(struct arm_smmu_device *smmu)
> +{
> +	return container_of(smmu, struct qcom_smmu, smmu);
> +}
> +
>  #define QCOM_ADRENO_SMMU_GPU_SID 0
> 
>  static bool qcom_adreno_smmu_is_gpu_device(struct device *dev)
> @@ -162,6 +168,50 @@ static const struct of_device_id
> qcom_smmu_client_of_match[] __maybe_unused = {
>  	{ }
>  };
> 
> +static int qcom_smmu_alloc_context_bank(struct arm_smmu_domain 
> *smmu_domain,
> +					struct arm_smmu_device *smmu,
> +					struct device *dev, int start)
> +{
> +	struct iommu_domain *domain = &smmu_domain->domain;
> +	struct qcom_smmu *qsmmu = to_qcom_smmu(smmu);
> +
> +	/* Keep identity domains as bypass, unless bypass is broken */
> +	if (domain->type == IOMMU_DOMAIN_IDENTITY && !qsmmu->bypass_broken)
> +		return ARM_SMMU_CBNDX_BYPASS;
> +
> +	/*
> +	 * The identity domain to emulate bypass is the only domain without a
> +	 * dev, use the last context bank for this to avoid collisions with
> +	 * active contexts during initialization.
> +	 */
> +	if (!dev)
> +		start = smmu->num_context_banks - 1;
> +
> +	return __arm_smmu_alloc_bitmap(smmu->context_map, start,
> smmu->num_context_banks);
> +}
> +
> +static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
> +{
> +	unsigned int last_s2cr = ARM_SMMU_GR0_S2CR(smmu->num_mapping_groups - 
> 1);
> +	struct qcom_smmu *qsmmu = to_qcom_smmu(smmu);
> +	u32 reg;
> +
> +	/*
> +	 * With some firmware writes to S2CR of type FAULT are ignored, and
> +	 * writing BYPASS will end up as FAULT in the register. Perform a 
> write
> +	 * to S2CR to detect if this is the case with the current firmware.
> +	 */
> +	reg = FIELD_PREP(ARM_SMMU_S2CR_TYPE, S2CR_TYPE_BYPASS) |
> +	      FIELD_PREP(ARM_SMMU_S2CR_CBNDX, 0xff) |
> +	      FIELD_PREP(ARM_SMMU_S2CR_PRIVCFG, S2CR_PRIVCFG_DEFAULT);
> +	arm_smmu_gr0_write(smmu, last_s2cr, reg);
> +	reg = arm_smmu_gr0_read(smmu, last_s2cr);
> +	if (FIELD_GET(ARM_SMMU_S2CR_TYPE, reg) != S2CR_TYPE_BYPASS)
> +		qsmmu->bypass_broken = true;
> +

Clever :)

Reviewed-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>

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