iommu/arm-smmu-v3: Explain why ATS stays disabled with bypass

The SMMU does not support enabling ATS for a bypass stream. Add a comment.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20230321100559.341981-1-jean-philippe@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>

drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c

@@ -2447,6 +2447,13 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
 
 	master->domain = smmu_domain;
 
+	/*
+	 * The SMMU does not support enabling ATS with bypass. When the STE is
+	 * in bypass (STE.Config[2:0] == 0b100), ATS Translation Requests and
+	 * Translated transactions are denied as though ATS is disabled for the
+	 * stream (STE.EATS == 0b00), causing F_BAD_ATS_TREQ and
+	 * F_TRANSL_FORBIDDEN events (IHI0070Ea 5.2 Stream Table Entry).
+	 */
 	if (smmu_domain->stage != ARM_SMMU_DOMAIN_BYPASS)
 		master->ats_enabled = arm_smmu_ats_supported(master);