linux-kernel - Re: [RFC PATCH] iommu/arm-smmu: Remove shutdown callback

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Date:   Mon, 8 Jun 2020 09:18:46 +0100
From:   Will Deacon <will@...nel.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [RFC PATCH] iommu/arm-smmu: Remove shutdown callback

On Sun, Jun 07, 2020 at 04:39:18PM +0530, Sai Prakash Ranjan wrote:
> Remove SMMU shutdown callback since it seems to cause more
> problems than benefits. With this callback, we need to make
> sure that all clients/consumers of SMMU do not perform any
> DMA activity once the SMMU is shutdown and translation is
> disabled. In other words we need to add shutdown callbacks
> for all those clients to make sure they do not perform any
> DMA or else we see all kinds of weird crashes during reboot
> or shutdown. This is clearly not scalable as the number of
> clients of SMMU would vary across SoCs and we would need to
> add shutdown callbacks to almost all drivers eventually.
> This callback was added for kexec usecase where it was known
> to cause memory corruptions when SMMU was not shutdown but
> that does not directly relate to SMMU because the memory
> corruption could be because of the client of SMMU which is
> not shutdown properly before booting into new kernel. So in
> that case, we need to identify the client of SMMU causing
> the memory corruption and add appropriate shutdown callback
> to the client rather than to the SMMU.
> 
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> ---
>  drivers/iommu/arm-smmu-v3.c | 6 ------
>  drivers/iommu/arm-smmu.c    | 6 ------
>  2 files changed, 12 deletions(-)

This feels like a giant bodge to me and I think that any driver which
continues to perform DMA after its ->shutdown() function has been invoked
is buggy. Wouldn't that cause problems with kexec(), for example?

There's a clear shutdown dependency ordering, where the clients of the
SMMU need to shutdown before the SMMU itself, but that's not really
the SMMU driver's problem to solve.

Will