iommu/arm-smmu-qcom: hide last LPASS SMMU context bank from linux

On qcom msm8998, writing to the last context bank of lpass_q6_smmu
(base address 0x05100000) produces a system freeze & reboot.

The hardware/hypervisor reports 13 context banks for the LPASS SMMU
on msm8998, but only the first 12 are accessible...
Override the number of context banks

[    2.546101] arm-smmu 5100000.iommu: probing hardware configuration...
[    2.552439] arm-smmu 5100000.iommu: SMMUv2 with:
[    2.558945] arm-smmu 5100000.iommu: 	stage 1 translation
[    2.563627] arm-smmu 5100000.iommu: 	address translation ops
[    2.568923] arm-smmu 5100000.iommu: 	non-coherent table walk
[    2.574566] arm-smmu 5100000.iommu: 	(IDR0.CTTW overridden by FW configuration)
[    2.580220] arm-smmu 5100000.iommu: 	stream matching with 12 register groups
[    2.587263] arm-smmu 5100000.iommu: 	13 context banks (0 stage-2 only)
[    2.614447] arm-smmu 5100000.iommu: 	Supported page sizes: 0x63315000
[    2.621358] arm-smmu 5100000.iommu: 	Stage-1: 36-bit VA -> 36-bit IPA
[    2.627772] arm-smmu 5100000.iommu: 	preserved 0 boot mappings

Specifically, the crashes occur here:

	qsmmu->bypass_cbndx = smmu->num_context_banks - 1;
	arm_smmu_cb_write(smmu, qsmmu->bypass_cbndx, ARM_SMMU_CB_SCTLR, 0);

and here:

	arm_smmu_write_context_bank(smmu, i);
	arm_smmu_cb_write(smmu, i, ARM_SMMU_CB_FSR, ARM_SMMU_CB_FSR_FAULT);

It is likely that FW reserves the last context bank for its own use,
thus a simple work-around is: DON'T USE IT in Linux.

If we decrease the number of context banks, last one will be "hidden".

Signed-off-by: Marc Gonzalez <mgonzalez@freebox.fr>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20240820-smmu-v3-1-2f71483b00ec@freebox.fr
Signed-off-by: Will Deacon <will@kernel.org>

0 files changed, 0 insertions, 0 deletions