linux-kernel - Re: [PATCH v2 4/8] iommu/arm-smmu-v3: move stall

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Date:   Tue, 1 Aug 2023 16:09:52 +0800
From:   Michael Shavit <mshavit@...gle.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, will@...nel.org,
        robin.murphy@....com, jgg@...dia.com, jean-philippe@...aro.org
Subject: Re: [PATCH v2 4/8] iommu/arm-smmu-v3: move stall_enabled to the cd table

> On Mon, Jul 31, 2023 at 09:28:40PM -0700, Nicolin Chen wrote:
> > This cd_table->stall_enabled comes from master->stall_enabled, and
> > cd_table will be in master structure. Also, struct arm_smmu_master
> > pointer will be passed in to arm_smmu_write_ctx_desc(). So, there
> > seems to be no need of master->cd_table.stall_enabled in the end;
> > just use master->stall_enabled directly?

Yes it's correct that this change isn't strictly necessary. Thoughts jgg@ ?

On Tue, Aug 1, 2023 at 12:52 PM Nicolin Chen <nicolinc@...dia.com> wrote:
> Actually the stall_enabled might still need to be per-CD/domain.
> If a domain is attached by two masters. The domain->stall_enabled
> is initialized with the first master->stall_enabled. Then, the
> second master->stall_enabled would be required to match with the
> domain->stall_enabled. arm_smmu_attach_dev() has such a sanity.
>
> So, I think we might not need this patch.

But why force domains attached to different masters to have the same
stall_enabled setting? Whether stall is enabled is strictly a property
of the master, not the domain. IMO the fact that it was stored in
domain and checked in attach_dev was only because the previous design
required it, not because it's more appropriate.