linux-kernel - Re: [PATCH] iommu: __iommu_attach_group: check for non-NULL blocking

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Message-ID: <aNzEOeCi8Zjn9S3N@valkosipuli.retiisi.eu>
Date: Wed, 1 Oct 2025 09:03:37 +0300
From: Sakari Ailus <sakari.ailus@....fi>
To: Hans Verkuil <hverkuil+cisco@...nel.org>
Cc: Jason Gunthorpe <jgg@...dia.com>, iommu@...ts.linux.dev,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH] iommu: __iommu_attach_group: check for non-NULL
 blocking_domain

Hi Hans, Jason,

On Mon, Sep 29, 2025 at 03:30:22PM +0200, Hans Verkuil wrote:
> On 29/09/2025 15:02, Jason Gunthorpe wrote:
> > On Mon, Sep 29, 2025 at 02:18:50PM +0200, Hans Verkuil wrote:
> >> On 29/09/2025 14:07, Jason Gunthorpe wrote:
> >>> On Mon, Sep 29, 2025 at 10:23:47AM +0200, Hans Verkuil wrote:
> >>>
> >>>> Since I am unfamiliar with the iommu core code, I am uncertain whether I am
> >>>> just papering over a bug elsewhere, or whether this is really the correct solution.
> >>>
> >>> It is papering over something, group->domain is not supposed to be
> >>> NULL at this point.. That probably means the iommu driver has not been
> >>
> >> It's group->blocking_domain that's NULL, not group->domain.
> > 
> > Er, I thought you were hitting a false positive on this:
> > 
> >   group->domain != group->blocking_domain
> > 
> > ie NULL != NULL
> > 
> > But I suppose the whole expression is checking for group->domain
> > already.
> > 
> > All your patch does is entirely disable the safetly logic :\
> > 
> > What is isp_attach_iommu() trying to accomplish? It does
> > arm_iommu_detach_device() and then arm_iommu_attach_device() ?
> > 
> > Why?
> > 
> > Is this trying to force a non-identity translation for ISP?

The omap3isp driver expects to use its own virtual address space for the
ISP: the video buffers are mapped there as virtually contiguous (physically
they can be whatever).

-- 
Kind regards,

Sakari Ailus