linux-kernel - [PATCH 2/2] iommu/arm-smmu-qcom: hide last context bank from linux

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Message-Id: <20240814-smmu-v1-2-3d6c27027d5b@freebox.fr>
Date: Wed, 14 Aug 2024 15:59:56 +0200
From: Marc Gonzalez <mgonzalez@...ebox.fr>
To: Rob Clark <robdclark@...il.com>, Will Deacon <will@...nel.org>, 
 Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>, 
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
 Conor Dooley <conor+dt@...nel.org>
Cc: iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 devicetree@...r.kernel.org, Bjorn Andersson <andersson@...nel.org>, 
 Konrad Dybcio <konradybcio@...nel.org>, Arnaud Vrac <avrac@...ebox.fr>, 
 Pierre-Hugues Husson <phhusson@...ebox.fr>, 
 Marijn Suijten <marijn.suijten@...ainline.org>, 
 Marc Gonzalez <mgonzalez@...ebox.fr>
Subject: [PATCH 2/2] iommu/arm-smmu-qcom: hide last 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.

Specifically, 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 would be: 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@...ebox.fr>
---
 drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
index 7e65189ca7b8c..e2e1fd9e2452b 100644
--- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
+++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
@@ -282,6 +282,11 @@ static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
 	u32 smr;
 	int i;
 
+	if (of_property_read_bool(smmu->dev->of_node, "qcom,last-ctx-bank-reserved")) {
+		dev_warn(smmu->dev, "hiding last ctx bank from linux");
+		--smmu->num_context_banks;
+	}
+
 	/*
 	 * Some platforms support more than the Arm SMMU architected maximum of
 	 * 128 stream matching groups. For unknown reasons, the additional

-- 
2.34.1