Re: [PATCH v2] firmware: qcom_scm: Only compile legacy calls on ARM

[Stephen Boyd <swboyd@chromium.org>]

Quoting Stephan Gerhold (2021-04-05 05:50:26)
> On Fri, Apr 02, 2021 at 10:21:58AM -0700, Stephen Boyd wrote:
> > 
> > Ah right, the whole secure world running in 32-bit mode thing. Is
> > msm8916 the only SoC that's using that? Or are there more? If only
> > msm8916 is affected then we could use a combination of CONFIG_ARM64 and
> > the compatible string to differentiate and then if more SoCs use 32-bit
> > secure world then we could have a new compatible string like qcom,scm-32
> > that tells us this fact. Maybe this was all discussed before and I
> > missed it. Either way, I'm trying to get rid of this boot call so that
> > we don't have to bounce to the firmware an extra time to figure out
> > something we can figure out from the kernel arch and scm compatible
> > string.
> 
> At least MSM8994 also uses SMC32 from what I heard. Overall it's
> probably quite hard to get that right now since all boards were tested
> with the dynamic detection so far. I suppose you could do the opposite,
> add an optional qcom,scm-64 to skip the detection step and force SMC64.

Isn't SMC64 going to be the overall majority going forward? Legacy
convention is for sure limited to CONFIG_ARM so I'll send another
follow-up patch to add a warning if we find legacy on CONFIG_ARM64.
SMC32 is hopefully no longer being produced given that it was introduced
at the time that the bootloader team wasn't supporting PSCI and didn't
want to support it. So we're making all new boards/SoCs/firmwares do
this calling convention probing to figure out something they already
know?

Maybe we should probe the calling convention on msm8994/msm8916 and
otherwise assume SMC64 on CONFIG_ARM64 kernels. I'd expect the exception
list to be smaller that way.

> 
> Also note that this could even be firmware-specific, not necessarily
> SoC-specific. There are some ancient MSM8916 firmwares that have legacy
> instead of SMC32. I could also imagine that there is also some SoC where
> there are different firmware versions with SMC32 or SMC64.

Sure but in theory the firmware would update the DT to indicate what
sort of firmware is there.

> 
> Plus, IMO the overhead for this detection is negligible. At least it
> ensures that we always use the right calling convention. The PSCI code
> probably does much more firmware calls to figure out all supported
> features.
> 

Heh, it tried to ensure we use the right calling convention but broke
things in the process, because the way of detecting the convention isn't
always there. I wouldn't be surprised if this comes up again for other
boards that use TF-A.