Re: arm-smmu-v3 sharing SID

[Robin Murphy <robin.murphy@arm.com>]

On 2023-05-18 11:46, Peng Fan wrote:
> Hi all,
> 
> Current arm-smmu-v3 driver does not support sharing SID,
> If there are two devices sharing one SID, the smmu-v3 driver
> will report "stream x already in tree".
> 
> We have an SOC that using smmu-v3, only supports limited
> SIDs. From my understanding, the smmu-v3 hardware
> supports SID sharing, but linux driver not support that.

Ugh, we've always hoped that nobody would build such a thing, since 
SMMUv3 allows for plenty of stream IDs for any reasonable system (Arm's 
implementations support at least 24 bits), and aliasing at the StreamID 
level does rather compromise the usefulness.

> I would like know is it ok to add support for sharing SID?
> Something like to use common rb_add/find, not
> smmu-v3 driver local rb tree add/find.

Not sure what you're thinking there - the StreamID tree is still private 
to the SMMU instance either way, and the existing arm_smmu_find_master() 
function is ideal for arm_smmu_device_group() to detect aliasing devices 
and group them appropriately. Then you also need some way to skip 
updating STEs that have already been touched for a previous device in 
the group (basically generalising what arm_smmu_install_ste_for_dev() 
currently does for duplicate StreamIDs owned by one device).

I'm fairly confident in reasoning about this for basic .attach_dev 
purposes, since I did implement the equivalent for SMMUv2, but I'm not 
sure I even want to think about what it would mean for SVA, PASIDs and 
nesting...

Thanks,
Robin.