iommu/arm-smmu: Improve SMR mask test

Make the SMR mask test more robust against SMR0 being live
at probe time, which might happen once we start supporting
firmware reservations for framebuffers and suchlike.
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

drivers/iommu/arm-smmu.c

@@ -971,23 +971,36 @@ static void arm_smmu_write_sme(struct arm_smmu_device *smmu, int idx)
 static void arm_smmu_test_smr_masks(struct arm_smmu_device *smmu)
 {
 	u32 smr;
+	int i;
 
 	if (!smmu->smrs)
 		return;
-
+	/*
+	 * If we've had to accommodate firmware memory regions, we may
+	 * have live SMRs by now; tread carefully...
+	 *
+	 * Somewhat perversely, not having a free SMR for this test implies we
+	 * can get away without it anyway, as we'll only be able to 'allocate'
+	 * these SMRs for the ID/mask values we're already trusting to be OK.
+	 */
+	for (i = 0; i < smmu->num_mapping_groups; i++)
+		if (!smmu->smrs[i].valid)
+			goto smr_ok;
+	return;
+smr_ok:
 	/*
 	 * SMR.ID bits may not be preserved if the corresponding MASK
 	 * bits are set, so check each one separately. We can reject
 	 * masters later if they try to claim IDs outside these masks.
 	 */
 	smr = FIELD_PREP(ARM_SMMU_SMR_ID, smmu->streamid_mask);
-	arm_smmu_gr0_write(smmu, ARM_SMMU_GR0_SMR(0), smr);
-	smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(0));
+	arm_smmu_gr0_write(smmu, ARM_SMMU_GR0_SMR(i), smr);
+	smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
 	smmu->streamid_mask = FIELD_GET(ARM_SMMU_SMR_ID, smr);
 
 	smr = FIELD_PREP(ARM_SMMU_SMR_MASK, smmu->streamid_mask);
-	arm_smmu_gr0_write(smmu, ARM_SMMU_GR0_SMR(0), smr);
-	smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(0));
+	arm_smmu_gr0_write(smmu, ARM_SMMU_GR0_SMR(i), smr);
+	smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
 	smmu->smr_mask_mask = FIELD_GET(ARM_SMMU_SMR_MASK, smr);
 }