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

[Nicolin Chen <nicolinc@nvidia.com>]

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
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

Thanks
Nicolin