Re: [PATCH v4 3/3] iommu/arm-smmu-qcom: Implement S2CR quirk

[Robin Murphy <robin.murphy@arm.com>]

On 2020-10-17 05:39, Bjorn Andersson wrote:
> The firmware found in some Qualcomm platforms intercepts writes to S2CR
> in order to replace bypass type streams with fault; and ignore S2CR
> updates of type fault.
> 
> Detect this behavior and implement a custom write_s2cr function in order
> to trick the firmware into supporting bypass streams by the means of
> configuring the stream for translation using a reserved and disabled
> context bank.
> 
> Also circumvent the problem of configuring faulting streams by
> configuring the stream as bypass.
> 
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
> 
> Changes since v3:
> - Move the reservation of the "identity context bank" to the Qualcomm specific
>    implementation.
> - Implement the S2CR quirk with the newly introduced write_s2cr callback.
> 
>   drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 68 ++++++++++++++++++++++
>   1 file changed, 68 insertions(+)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> index 0089048342dd..c0f42d6a6e01 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> @@ -10,8 +10,14 @@
>   
>   struct qcom_smmu {
>   	struct arm_smmu_device smmu;
> +	bool bypass_cbndx;

Nit: variables named "*ndx" usually hold an actual index value. If it's 
just a flag then maybe name it something like "use_bypass_context"?

>   };
>   
> +static struct qcom_smmu *to_qcom_smmu(struct arm_smmu_device *smmu)
> +{
> +	return container_of(smmu, struct qcom_smmu, smmu);
> +}
> +
>   static const struct of_device_id qcom_smmu_client_of_match[] __maybe_unused = {
>   	{ .compatible = "qcom,adreno" },
>   	{ .compatible = "qcom,mdp4" },
> @@ -25,9 +31,32 @@ static const struct of_device_id qcom_smmu_client_of_match[] __maybe_unused = {
>   
>   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;
>   	u32 smr;
>   	int i;
>   
> +	/*
> +	 * With some firmware versions writes to S2CR of type FAULT are
> +	 * ignored, and writing BYPASS will end up written as FAULT in the
> +	 * register. Perform a write to S2CR to detect if this is the case and
> +	 * if so reserve a context bank to emulate bypass streams.
> +	 */
> +	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_cbndx = smmu->num_context_banks - 1;

Oh, so maybe the name is in fact OK but the type is wrong :/

I guess this does happens to work out, but for the wrong reason...

> +
> +		set_bit(qsmmu->bypass_cbndx, smmu->context_map);
> +
> +		reg = FIELD_PREP(ARM_SMMU_CBAR_TYPE, CBAR_TYPE_S1_TRANS_S2_BYPASS);
> +		arm_smmu_gr1_write(smmu, ARM_SMMU_GR1_CBAR(qsmmu->bypass_cbndx), reg);
> +	}
> +
>   	for (i = 0; i < smmu->num_mapping_groups; i++) {
>   		smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
>   
> @@ -46,6 +75,44 @@ static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
>   	return 0;
>   }
>   
> +static void qcom_smmu_write_s2cr(struct arm_smmu_device *smmu, int idx)
> +{
> +	struct arm_smmu_s2cr *s2cr = smmu->s2crs + idx;
> +	struct qcom_smmu *qsmmu = to_qcom_smmu(smmu);
> +	u32 cbndx = s2cr->cbndx;
> +	u32 type = s2cr->type;
> +	u32 reg;
> +
> +	if (qsmmu->bypass_cbndx) {

Note that if we are talking indices here then 0 would be perfectly valid 
in general. This works out OK in practice given that we're always 
reserving the last implemented context above, and if we ever *did* only 
have one such that index 0 is the last then we're going to have a bad 
time either way, but it's not necessarily the most obvious.

> +		if (type == S2CR_TYPE_BYPASS) {
> +			/*
> +			 * Firmware with quirky S2CR handling will substitute
> +			 * BYPASS writes with FAULT, so point the stream to the
> +			 * reserved context bank and ask for translation on the
> +			 * stream
> +			 */
> +			type = S2CR_TYPE_TRANS;
> +			cbndx = qsmmu->bypass_cbndx;
> +		} else if (type == S2CR_TYPE_FAULT) {
> +			/*
> +			 * Firmware with quirky S2CR handling will ignore FAULT
> +			 * writes, so trick it to write FAULT by asking for a
> +			 * BYPASS.
> +			 */
> +			type = S2CR_TYPE_BYPASS;

Ha, that's brilliant :)

> +			cbndx = 0xff;
> +		}
> +	}
> +
> +	reg = FIELD_PREP(ARM_SMMU_S2CR_TYPE, type) |
> +	      FIELD_PREP(ARM_SMMU_S2CR_CBNDX, cbndx) |
> +	      FIELD_PREP(ARM_SMMU_S2CR_PRIVCFG, s2cr->privcfg);
> +
> +	if (smmu->features & ARM_SMMU_FEAT_EXIDS && smmu->smrs && smmu->smrs[idx].valid)
> +		reg |= ARM_SMMU_S2CR_EXIDVALID;

Does any of your hardware actually have EXIDS implemented? No big deal 
if you only want this here "just in case", I'm just curious as I was 
under then impression that it was essentially a ThunderX special.

Other than sorting out bypass_cbndx one way or the other, overall this 
is now looking about as nice as it ever could - thanks for persevering!

Robin.

> +	arm_smmu_gr0_write(smmu, ARM_SMMU_GR0_S2CR(idx), reg);
> +}
> +
>   static int qcom_smmu_def_domain_type(struct device *dev)
>   {
>   	const struct of_device_id *match =
> @@ -87,6 +154,7 @@ static const struct arm_smmu_impl qcom_smmu_impl = {
>   	.cfg_probe = qcom_smmu_cfg_probe,
>   	.def_domain_type = qcom_smmu_def_domain_type,
>   	.reset = qcom_smmu500_reset,
> +	.write_s2cr = qcom_smmu_write_s2cr,
>   };
>   
>   struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu)
>