Re: [PATCH v5 5/6] dma: swiotlb: Add an overridable architecture pool opt-out

From: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>,
	iommu@lists.linux.dev, linux-kernel@vger.kernel.org
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
	Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Steven Price <steven.price@arm.com>,
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Subject: Re: [PATCH v5 5/6] dma: swiotlb: Add an overridable architecture pool opt-out
Date: Wed, 23 Sep 2026 19:48:13 +0530	[thread overview]
Message-ID: <yq5azex8126y.fsf@kernel.org> (raw)
In-Reply-To: <8c9c04ae-cba7-4096-a7d1-70b90a46c161@arm.com>

Robin Murphy <robin.murphy@arm.com> writes:

> On 21/09/2026 7:36 am, Aneesh Kumar K.V (Arm) wrote:
>> On x86, iommu=off historically prevented SWIOTLB initialization by not
>> setting SWIOTLB_INIT_ADDRESSING_LIMIT. This overloads that flag: it
>> controls both whether a pool is selected and where the pool is
>> allocated. If another requirement, swiotlb=force selects a pool
>> independently, omitting SWIOTLB_INIT_ADDRESSING_LIMIT may place it above
>> the addressable range of devices which need bounce buffering.
>
> Well then surely something is wrong with the design? If 
> SWIOTLB_INIT_ADDRESSING_LIMIT is set then SWIOTLB still needs to respect 
> it irrespective of any *additional* reasons for activating (either from 
> arch code or internally). And conversely, if devices have addressing 
> limits but SWIOTLB_INIT_ADDRESSING_LIMIT is *not* set, then the arch 
> code has clearly gone wrong or made a bad assumption, but that doesn't 
> seem like SWIOTLB's problem to worry about.
>

That is what this patch does. It sets SWIOTLB_INIT_ADDRESSING_LIMIT
based on max_possible_pfn, while SWIOTLB_INIT_FORCE_DISABLE allows the
architecture to opt out of SWIOTLB initialization independently.

>
>> Add SWIOTLB_INIT_FORCE_DISABLE to let an architecture opt out of normal
>> pool initialization without discarding the pool placement constraints.
>> 
>> Unlike swiotlb_force_disable, which is set by swiotlb=noforce and
>> unconditionally prevents pool initialization, the new flag is an
>> architecture default. Explicit requirements such as memory encryption,
>> architecture remapping, or swiotlb=force take precedence over it.
>
> Logically, that should already be implied by the architecture passing no 
> SWIOTLB_INIT flags.
>

We now have a case where max_possible_pfn causes
SWIOTLB_INIT_ADDRESSING_LIMIT to be set, while the kernel command line
specifies iommu=off. We want to disable SWIOTLB in this case even though
the SWIOTLB_INIT flags are nonzero.

-aneesh