linux-kernel - Re: [PATCH v7 4/6] media: verisilicon: AV1: Restore IOMMU context before decoding a frame

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Message-ID: <20250825183122.GB1899851@ziepe.ca>
Date: Mon, 25 Aug 2025 15:31:22 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Nicolas Dufresne <nicolas.dufresne@...labora.com>
Cc: Benjamin Gaignard <benjamin.gaignard@...labora.com>, joro@...tes.org,
	will@...nel.org, robin.murphy@....com, robh@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, heiko@...ech.de,
	p.zabel@...gutronix.de, mchehab@...nel.org, iommu@...ts.linux.dev,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org, kernel@...labora.com,
	linux-media@...r.kernel.org
Subject: Re: [PATCH v7 4/6] media: verisilicon: AV1: Restore IOMMU context
 before decoding a frame

On Mon, Aug 25, 2025 at 01:50:16PM -0400, Nicolas Dufresne wrote:

> Jason, the point is that the iommu and the VPU are not separate devices, which
> comes with side effects. On RKVDec side, the iommu configuration get resets
> whenever a decoding error leads to a VPU "self reset". I can't remember who from
> the iommu subsystem suggested that, but the empty domain method was agreed to be

IDK, that seems really goofy too me an defiantly needs to be
extensively documented this is restoring the default with some lore
link of the original suggestion.

> the least invasive way to workaround that issue. I believe Detlev tried multiple
> time to add APIs for that before the discussion lead to this path.

You mean this:

https://lore.kernel.org/linux-iommu/20250318152049.14781-1-detlev.casanova@collabora.com/

Which came back with the same remark I would give:

 Please have some kind of proper reset notifier mechanism - in fact
 with runtime PM could you not already invoke a suspend/resume cycle
 via the device links?

Or another reasonable option:

  Or at worst just export a public interface for the other driver to
  invoke rk_iommu_resume() directly.

Sigh.

> Benjamin, please improve the naming, comment and description, I agree with Jason
> its not completely clear. I'm also surprised that you do that every frame, seems
> excessive.

Indeed if it is just error recovery.

> [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ff8c5622f9f7c644e995d013af320b59e4d61b93

This is already merged? And now you want two copies of this? I think
this is a very poor direction..

Jason