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

[Pranjal Shrivastava <praan@google.com>]

On Tue, Oct 15, 2024 at 09:47:23AM -0300, 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.
> 
> If this is a common thing we could have the core code take on more of
> the job.

Yes! I've seen the IOMMU being bypassed at multiple places, primarily
for performance, people like bypassing the iommu for "trusted" devices.
A few examples that are publically accessible: Qcomm SoCs [1], [2].
Seems like Qualcomm have a DT property `qcomm-s1-bypass` to achieve
something similar.

In fact, *blast from the past*, I tried to do something similar sometime
ago with [3]. Although, perhaps that wasn't the best way (and I was a
kernel newbie :))

A little off-topic, but I think there has been some interest to bypass
the default substream as well while still maintaining PASID isolation.[4]

Although, as far as arm-smmu-v3 is concerned, IIRC, I think there was a
way to tell that the region is reserved and don't map it.

> 
> Jason

Thanks,
Pranjal

[1]
https://github.com/realme-kernel-opensource/realme5-kernel-source/blob/master/arch/arm64/boot/dts/qcom/sa8155-vm-qupv3.dtsi#L22

[2] 
https://android.googlesource.com/kernel/msm/+/android-7.1.0_r0.2/Documentation/devicetree/bindings/platform/msm/ipa.txt#28

[3]
https://lore.kernel.org/all/20230707104857.348353-1-praan@google.com/

[4]
https://lore.kernel.org/all/CAGfWUPziSWNMc_px4E-i+_V_Jxdb_WSwOLXHZ+PANz2Tv5pFPA@mail.gmail.com/