linux-kernel - RE: [PATCH RFC 0/2] iommu/arm-smmu-v3: bypass streamid zero on i.MX95

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Message-ID:
 <PAXPR04MB8459394682B0B3342FA0383388462@PAXPR04MB8459.eurprd04.prod.outlook.com>
Date: Wed, 16 Oct 2024 08:53:20 +0000
From: Peng Fan <peng.fan@....com>
To: Nicolin Chen <nicolinc@...dia.com>
CC: Robin Murphy <robin.murphy@....com>, "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"
	<linux-arm-kernel@...ts.infradead.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Jason Gunthorpe <jgg@...pe.ca>
Subject: RE: [PATCH RFC 0/2] iommu/arm-smmu-v3: bypass streamid zero on i.MX95

> Subject: Re: [PATCH RFC 0/2] iommu/arm-smmu-v3: bypass streamid
> zero on i.MX95
> 
> On Wed, Oct 16, 2024 at 12:56:54AM +0000, Peng Fan wrote:
> > > Subject: Re: [PATCH RFC 0/2] iommu/arm-smmu-v3: bypass
> streamid zero
> > > on i.MX95
> > >
> > > On 2024-10-15 4:14 am, Peng Fan (OSS) wrote:
> > > > i.MX95 eDMA3 connects to DSU ACP, supporting dma coherent
> > > memory to
> > > > memory operations. However TBU is in the path between eDMA3
> > > and ACP,
> > > > need to bypass the default SID 0 to make eDMA3 work properly.
> > >
> > > I'm confused, why not just describe that the device owns this
> > > StreamID in the DT the normal way, i.e, "iommus = <&smmu 0>;"?
> >
> > Current SMMU-v3 driver not support streamID sharing, but we have
> > limited streamIDs(i.MX95 max supports 64 SIDs). However the eDMA
> > supports 64 channels, each channel may have a SID, but not
> supported
> > by current smmuv3 driver.
> >
> > We see smmu would degrade the performance in somecases, so we
> wanna to
> > give user an option to bypass SMMU for eDMA.
> 
> If it's a system-wise global need, adding "iommu.passthrough=y"
> to the bootargs string should work for all trusted devices. And

No. not system wide.

> particular group can be changed to the IDENTITY domain too upon
> cmdline setting, e.g.
> echo identity | sudo tee
> /sys/class/pci_bus/0009\:01/device/iommu_group/type

This means I still need to put iommus = <&smmu 0> under
the device tree node.

Thanks,
Peng.

> 
> Thanks
> Nicolin