linux-kernel - Re: [PATCH 3/7] of/iommu: add multi-map support

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Message-ID: <78ffd1ce-5408-4bf5-a782-0c4ca14017b1@arm.com>
Date: Tue, 27 Jan 2026 14:20:09 +0000
From: Robin Murphy <robin.murphy@....com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
 Vikash Garodia <vikash.garodia@....qualcomm.com>
Cc: Dikshita Agarwal <dikshita.agarwal@....qualcomm.com>,
 Abhinav Kumar <abhinav.kumar@...ux.dev>, Bryan O'Donoghue <bod@...nel.org>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Saravana Kannan <saravanak@...nel.org>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Stefan Schmidt <stefan.schmidt@...aro.org>,
 Hans Verkuil <hverkuil@...nel.org>, Krzysztof Kozlowski <krzk@...nel.org>,
 Vishnu Reddy <busanna.reddy@....qualcomm.com>,
 Hans Verkuil <hverkuil+cisco@...nel.org>, linux-arm-msm@...r.kernel.org,
 linux-media@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
 Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
 Charan Teja Kalla <charan.kalla@....qualcomm.com>,
 Vijayanand Jitta <vijayanand.jitta@....qualcomm.com>
Subject: Re: [PATCH 3/7] of/iommu: add multi-map support

On 2026-01-27 11:45 am, Dmitry Baryshkov wrote:
> On Mon, Jan 26, 2026 at 05:55:46PM +0530, Vikash Garodia wrote:
>> From: Charan Teja Kalla <charan.kalla@....qualcomm.com>
>>
>> When multiple mappings are present for an input id, linux matches just
>> the first one. There is a usecase[1] where all the mappings are to be
>> maintained in parallel for an iommu-map entry of a same input id.
> 
> This contradicts the IOMMU idealogy (at least as far as I understood it
> fom the maintainers): the device (driver) doesn't control which IOMMUs
> are getting used. Instead _all_ defined entries should get used. For
> iommu-map it means that if the map defines several entries for a single
> function, then all entries should always get mapped.

Indeed there is no concept of "multi-map" - if a single input ID 
represents more than one thing then that notion of "input ID" is 
fundamentally wrong. A single *device* may have multiple IDs, as in the 
case of PCI bridge aliasing, but in that case there are multiple things 
to map.

Thanks,
Robin.