linux-kernel - Re: [PATCH RFC 2/2] iommu/arm-smmu-v3: Bypass SID0 for NXP i.MX95

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Message-ID: <Zw6H_fl14_r3bvgw@google.com>
Date: Tue, 15 Oct 2024 15:19:25 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Robin Murphy <robin.murphy@....com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, "Peng Fan (OSS)" <peng.fan@....nxp.com>,
	Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Joy Zou <joy.zou@....com>,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peng Fan <peng.fan@....com>
Subject: Re: [PATCH RFC 2/2] iommu/arm-smmu-v3: Bypass SID0 for NXP i.MX95

On Tue, Oct 15, 2024 at 04:13:13PM +0100, Robin Murphy wrote:
> On 2024-10-15 1:47 pm, Jason Gunthorpe wrote:
> > On Tue, Oct 15, 2024 at 08:13:28AM +0000, Pranjal Shrivastava wrote:
> > 
> > > Umm.. this was specific for rmr not a generic thing. I'd suggest to
> > > avoid meddling with the STEs directly for acheiving bypass. Playing
> > > with the iommu domain type could be neater. Perhaps, modify the
> > > ops->def_domain_type to return an appropriate domain?
> > 
> > Yeah, that is the expected way, to force the def_domain_type to
> > IDENTITY and refuse to attach a PAGING/BLOCKED domain.
> 
> There is no domain, this is bypassing an arbitrary StreamID not associated
> with any device. Which incidentally is something an IORT RMR can quite
> happily achieve already (I think the DT reserved-memory binding does need a
> proper device node to relate to, though).

+1. I assumed that the use-case was to first attach the streamID to a
device and then intall a bypass for that specific streamID. If that's
not the case, I'm not sure why are we trying to achieve that.

I thought about rmr too, but it looks like the "device" is a DMA and may
want to write to more than a fixed region of memory.

> 
> Thanks,
> Robin.

Thanks,
Pranjal