linux-kernel - Re: [PATCH v2 4/4] iommu: Get DT/ACPI parsing into the proper probe path

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Message-ID: <aAkj5P1I-e9lylIU@google.com>
Date: Wed, 23 Apr 2025 10:31:16 -0700
From: William McVicker <willmcvicker@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Robin Murphy <robin.murphy@....com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Hanjun Guo <guohanjun@...wei.com>,
	Sudeep Holla <sudeep.holla@....com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Len Brown <lenb@...nel.org>, Russell King <linux@...linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Stuart Yoder <stuyoder@...il.com>,
	Laurentiu Tudor <laurentiu.tudor@....com>,
	Nipun Gupta <nipun.gupta@....com>,
	Nikhil Agarwal <nikhil.agarwal@....com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-acpi@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev, devicetree@...r.kernel.org,
	linux-pci@...r.kernel.org,
	Charan Teja Kalla <quic_charante@...cinc.com>
Subject: Re: [PATCH v2 4/4] iommu: Get DT/ACPI parsing into the proper probe
 path

On 04/22/2025, Jason Gunthorpe wrote:
> On Tue, Apr 22, 2025 at 02:55:28PM -0700, William McVicker wrote:
> 
> > On this note, I was looking through `of_dma_configure_id()` and am also
> > wondering if we may hit other race conditions if the device is still being
> > probed and the dma properties (like the coherent dma mask) haven't been fully
> > populated? Just checking if the driver is bound, doesn't seem like enough to
> > start configuring the DMA when async probing can happen.
> 
> I think the reasoning at work here is that the plugin path for a
> struct device should synchronously setup the iommu.
> 
> There is enough locking there that the iommu code won't allow the
> device plugin to continue until the iommu is fully setup under the
> global lock.
> 
> The trick of using dev->driver is only a way to tell if this function
> is being called from the driver plugin path just before starting the
> driver, or from the iommu code just before configuring the iommu.
> 
> Given that explanation can you see issues with of_dma_configure_id() ?
> 
> Jason

I think the only concern is when a driver calls dma_set_mask_and_coherent() in
it's probe function. If we can handle that case in an asynchrounous manner,
then I think we are good.

Thanks,
Will