Re: [PATCH] xen/arm: smmuv1: Revert associating the group pointer with the S2CR

[Julien Grall <julien@xxxxxxx>]

On 16/04/2021 16:01, Rahul Singh wrote:
> Hi Julien,
Hi Rahul,

> > On 16 Apr 2021, at 3:35 pm, Julien Grall <julien@xxxxxxx> wrote:
> >
> > Hi,
> >
> > On 16/04/2021 12:25, Rahul Singh wrote:
> > > Revert the code that associates the group pointer with the S2CR as this
> > > code causing an issue when the SMMU device has more than one master
> > > device.
> > It is not clear to me why this change was first added. Are we missing any 
> > feature when reverting it?
> This feature was added when we backported the code from Linux to fix the stream 
> match conflict issue
> as part of commit "xen/arm: smmuv1: Intelligent SMR allocation”.
>
> This is an extra feature added to allocate IOMMU group based on stream-id. If 
> two device has the
> same stream-id then we assign those devices to the same group. 
If we revert the patch, then it would not be possible to use the SMMU if 
two devices use the same stream-id. Is that correct?
> This code was removed from Linux
> later point in time when IOMMU group handling is done by Linux code not by a 
> specific IOMMU driver.
Right.... But Linux still support that option. Is that correct?

> Therefore I think it is ok revert the code.
I am ok with the principle of (partially) reverting patch to unblock the 
situation. But I have to admit, I don't quite understand why this is 
reverted rather than fixed.
Cheers,

--
Julien Grall