linux-kernel - Re: [PATCH] iommu/vt-d: Remove an unnecessary call set_dma

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Message-ID: <9bd6cbaa-3a1f-4cc5-8581-3a99dc6ac2c2@arm.com>
Date: Thu, 10 Apr 2025 12:33:51 +0100
From: Robin Murphy <robin.murphy@....com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Petr Tesařík
 <petr@...arici.cz>, iommu@...ts.linux.dev
Cc: Petr Tesarik <ptesarik@...e.com>, David Woodhouse <dwmw2@...radead.org>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Tom Murphy <murphyt7@....ie>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/vt-d: Remove an unnecessary call set_dma_ops()

On 10/04/2025 3:59 am, Baolu Lu wrote:
> On 4/9/25 23:44, Petr Tesařík wrote:
>> On Thu,  3 Apr 2025 18:56:05 +0200
>> Petr Tesarik<ptesarik@...e.com> wrote:
>>
>>> Do not touch per-device DMA ops when the driver has been converted to 
>>> use
>>> the dma-iommu API.
>>>
>>> Fixes: c588072bba6b ("iommu/vt-d: Convert intel iommu driver to the 
>>> iommu ops")
>>> Signed-off-by: Petr Tesarik<ptesarik@...e.com>
>> Does anyone agree? Or see any issues?

At least it definitely serves no purpose since b5c58b2fdc42 
("dma-mapping: direct calls for dma-iommu"). FWIW,

Reviewed-by: Robin Murphy <robin.murphy@....com>

> This change looks good to me. The iommu driver should not call
> set_dma_ops() anyway. I will queue this for v6.15-rc.
> 
> I ever wondered whether we could clean up dev->dma_iommu when the device
> is released by the iommu core. But, iommu_release_device() is only
> called in the device removal path, so keeping dev->dma_iommu set doesn't
> impose any functional impact AFAICS.

Funnily enough I've just been looking at that - I'd long held the same 
assumption, but I'd been forgetting the edge case that's always been 
there if bus_set_iommu(), and now iommu_device_register(), failed and 
groups and default domains were torn down for existing devices. So in 
fact it seems Intel was one of the few drivers/architectures managing to 
do the right thing before :)

Cheers,
Robin.