linux-kernel - Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev

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Message-ID: <aKf7/Fk/dT6UZWb9@Asurada-Nvidia>
Date: Thu, 21 Aug 2025 22:11:24 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <robin.murphy@....com>, <joro@...tes.org>, <bhelgaas@...gle.com>,
	<will@...nel.org>, <robin.clark@....qualcomm.com>, <yong.wu@...iatek.com>,
	<matthias.bgg@...il.com>, <angelogioacchino.delregno@...labora.com>,
	<thierry.reding@...il.com>, <vdumpa@...dia.com>, <jonathanh@...dia.com>,
	<rafael@...nel.org>, <lenb@...nel.org>, <kevin.tian@...el.com>,
	<yi.l.liu@...el.com>, <baolu.lu@...ux.intel.com>,
	<linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
	<linux-mediatek@...ts.infradead.org>, <linux-tegra@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>, <linux-pci@...r.kernel.org>,
	<patches@...ts.linux.dev>, <pjaroszynski@...dia.com>, <vsethi@...dia.com>,
	<helgaas@...nel.org>, <etzhao1900@...il.com>
Subject: Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked()
 helper

On Thu, Aug 21, 2025 at 03:37:04PM -0300, Jason Gunthorpe wrote:
> On Thu, Aug 21, 2025 at 08:22:06AM -0700, Nicolin Chen wrote:
> > I should have pasted the full piece:
> > drivers/gpu/drm/tegra/drm.c-960-	/*
> > drivers/gpu/drm/tegra/drm.c:961:	 * If the host1x client is already attached to an IOMMU domain that is
> > drivers/gpu/drm/tegra/drm.c-962-	 * not the shared IOMMU domain, don't try to attach it to a different
> > drivers/gpu/drm/tegra/drm.c-963-	 * domain. This allows using the IOMMU-backed DMA API.
> > drivers/gpu/drm/tegra/drm.c-964-	 */
> > drivers/gpu/drm/tegra/drm.c-965-	if (domain && domain->type != IOMMU_DOMAIN_IDENTITY &&
> > drivers/gpu/drm/tegra/drm.c-966-	    domain != tegra->domain)
> > 
> > So, the check is two-fold:
> > 1) is attached
> > 2) is the shared IOMMU domain (tegra->domain?)
> 
> Yea
> 
>  iommu_is_domain_currently_attached(dev, tegra->domain)

Ah, yea.

> > Overall, I feel this would be a big project yet arguably for a low
> > reward..
> 
> Indeed, we can drop a FIXME comment there and leave it as the last
> user or something perhaps

I see. We could keep it in the library but discourage new callers.

I will start with the internal cleanup as we discussed, as a part
of this PCI reset series. Then, I will try adding those new helper
functions that we've listed here, as a separate series.

Thanks
Nicolin