Re: [PATCH 2/6] iommu/qcom: Write TCR before TTBRs to fix ASID access behavior

Marijn Suijten
2022-06-06T00:55:43Z

On 2022-05-31 16:55:59, Will Deacon wrote:
> On Fri, May 27, 2022 at 11:28:57PM +0200, Konrad Dybcio wrote:
> > From: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
> >
> > As also stated in the arm-smmu driver, we must write the TCR before
> > writing the TTBRs, since the TCR determines the access behavior of
> > some fields.
>
> Where is this stated in the arm-smmu driver?
>
> >
> > Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
> > Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
> > Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
> > ---
> > drivers/iommu/arm/arm-smmu/qcom_iommu.c | 12 ++++++------
> > 1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/iommu/arm/arm-smmu/qcom_iommu.c b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> > index 1728d4d7fe25..75f353866c40 100644
> > --- a/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> > +++ b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> > @@ -273,18 +273,18 @@ static int qcom_iommu_init_domain(struct iommu_domain *domain,
> > ctx->secure_init = true;
> > }
> >
> > - /* TTBRs */
> > - iommu_writeq(ctx, ARM_SMMU_CB_TTBR0,
> > - pgtbl_cfg.arm_lpae_s1_cfg.ttbr |
> > - FIELD_PREP(ARM_SMMU_TTBRn_ASID, ctx->asid));
> > - iommu_writeq(ctx, ARM_SMMU_CB_TTBR1, 0);
> > -
> > /* TCR */
> > iommu_writel(ctx, ARM_SMMU_CB_TCR2,
> > arm_smmu_lpae_tcr2(&pgtbl_cfg));
> > iommu_writel(ctx, ARM_SMMU_CB_TCR,
> > arm_smmu_lpae_tcr(&pgtbl_cfg) | ARM_SMMU_TCR_EAE);
> >
> > + /* TTBRs */
> > + iommu_writeq(ctx, ARM_SMMU_CB_TTBR0,
> > + pgtbl_cfg.arm_lpae_s1_cfg.ttbr |
> > + FIELD_PREP(ARM_SMMU_TTBRn_ASID, ctx->asid));
> > + iommu_writeq(ctx, ARM_SMMU_CB_TTBR1, 0);
>
> I'd have thought that SCTLR.M would be clear here, so it shouldn't matter
> what order we write these in.

Having tested the series without this particular patch on 8976 (Sony
Loire Suzu), it doesn't seem to matter indeed. I'll ask around if this
"access behaviour" was observed on a different board/platform.

- Marijn