Re: [PATCH 1/6] iommu/qcom: Use the asid read from device-tree if specified

[Rob Clark]

On Tue, May 31, 2022 at 9:19 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Tue, May 31, 2022 at 09:15:22AM -0700, Rob Clark wrote:
> > On Tue, May 31, 2022 at 8:46 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > >
> > > On Fri, May 27, 2022 at 11:28:56PM +0200, Konrad Dybcio wrote:
> > > > From: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxxx>
> > > >
> > > > As specified in this driver, the context banks are 0x1000 apart.
> > > > Problem is that sometimes the context number (our asid) does not
> > > > match this logic and we end up using the wrong one: this starts
> > > > being a problem in the case that we need to send TZ commands
> > > > to do anything on a specific context.
> > >
> > > I don't understand this. The ASID is a software construct, so it shouldn't
> > > matter what we use. If it does matter, then please can you explain why? The
> > > fact that the context banks are 0x1000 apart seems unrelated.
> >
> > I think the connection is that mapping from ctx bank to ASID is 1:1
>
> But in what sense? How is the ASID used beyond a tag in the TLB? The commit
> message hints at "TZ commands" being a problem.
>
> I'm not doubting that this is needed to make the thing work, I just don't
> understand why.

(disclaimer, it has been quite a while since I've looked at the smmu
setup with earlier tz, ie. things that use qcom_iommu, but from
memory...)

We cannot actually assign the context banks ourselves, so in the dt
bindings the "ASID" is actually the context bank index.  I don't
remember exactly if this was a limitation of the tz interface, or
result of not being able to program the smmu's global registers
ourselves.

BR,
-R