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

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Message-ID: <20250929130214.GK2617119@nvidia.com>
Date: Mon, 29 Sep 2025 10:02:14 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Hans Verkuil <hverkuil+cisco@...nel.org>
Cc: 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>
Subject: Re: [PATCH] iommu: __iommu_attach_group: check for non-NULL
 blocking_domain

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?

Jason