Re: [PATCH v2] arm64: dts: foundation-v8: Enable PSCI mode

[Daniel Thompson]

On 20/09/17 10:42, Sudeep Holla wrote:
> On 19/09/17 19:32, Daniel Thompson wrote:
> > Currently if the Foundation model is running ARM Trusted Firmware then
> > the kernel, which is configured to use spin tables, cannot start secondary
> > processors or "power off" the simulation.
> >
> > After adding a couple of labels to the include file and splitting out the
> > spin-table configuration into a header, we add a couple of new headers
> > together with two new DTs (GICv2+PSCI and GICv3+PSCI).
> >
> > The new GICv3+PSCI DT has been boot tested, the remaining three (two of
> > which existed prior to this patch) have been "tested" by decompiling the
> > blobs and comparing them against a reference.
>
> How different are these from the ones hosted in [1] ?

They look like they were either independently written or diverged a long 
time ago. The existing kernel DTs describe hardware absent from the ARM 
TF ones and vice versa.

With specific reference to PSCI it looks like my patches could perhaps 
be improved by adding idle-state support.


> On argument is that we want to take the DTS out of device tree as
> firmware is responsible for generating them. Alternatively, we may be
> duplicating resulting in discrepancies over time by coping it into kernel.

The general problem is copying from where?

The kernel DTs are a well maintained centralized repository which is 
*really* useful. git grep across the kernel DTs is a hugely powerful 
tool when trying to better understand an ecosystem as sprawling and 
diverse as ARMs. In fact I've even seen those sort of searchs used as a 
basis to clean up unused code. Seeing that centralized repository 
splinter into separate per-vendor silos would be a huge loss for kernel 
developers.


> Since users of ARM TF must be able to access these, I am not sure if it
> makes sense to merge these. Or we remove it from ARM TF to avoid any
> conflicts/discrepancies. >
> Thoughts ?

I kind of agree that maintaining DTs and DT documentation in the kernel 
is a little odd given that the kernel is not the only player here 
(FreeBSD, u-boot, etc). However it is sufficiently well maintained that 
projects are content(ish) to regard the kernel as the canonical source 
for these things (u-boot, for example, seeks to shadow kernel DTs 
without modifying them).

However regardless of the above I'd say they should be removed from ARM 
TF. ARM TF does not use, modify, pass on or in any way consume DT... it 
has no skin in the game here. Why does it want to own a few of blobs for 
a small subset of the platforms it supports? I'm afraid that makes no 
sense to me, to the extent that it didn't even occur to me to *look* in 
the ARM TF sources to find any DTs for FVP until you pointed them out.

In other words, whilst people could discuss alternative ways to manage 
DTs[1], I can't see any universe where ARM TF would be a logical place 
to keep them.


Daniel.


[1] ... and I'd further suggest that only perhaps people who are
    prepared to put resources into fixing it should convene such a
    discussion.