Re: [PATCH 1/3] devicetree: bindings: Document qcom board compatible format

[Stephen Boyd]

On 11/12, Rob Herring wrote:
> On Thu, Nov 12, 2015 at 1:44 PM, Stephen Boyd <sboyd@xxxxxxxxxxxxxx> wrote:
> > On 11/12, Rob Herring wrote:
> >> On Mon, Oct 26, 2015 at 02:25:10PM -0700, Stephen Boyd wrote:
> 
> >> > +Some qcom based bootloaders identify the dtb blob based on a set of
> >> > +device properties like SoC, platform, PMIC, and revisions of those components.
> >> > +To support this scheme, we encode this information into the board compatible
> >> > +string.
> >>
> >> Why does all this need to be a single property?
> >
> > Because the different vendor properties were rejected by arm-soc
> > maintainers and the board compatible string was suggested as the
> > place to put such information.
> 
> I'm surprised an 80+ character compatible stream is okay. The parts
> giving me heartburn here are not mentioned in the previous discussion
> nor the QCDT format.
> 
> As presented previously I agree with the push back. However, we could
> do standard properties for SOC and board versions rather than
> something vendor specific. Then the existing kernel support for
> versions could use it. We could also just make this compatible string
> formatting standard for more than just QC boards.

Some standard properties for these things sounds good to me.
What's the existing kernel support for versions though? Is that
just compatible string matching, or something else?

> 
> >> > +
> >> > +The 'soc_version', 'plat_version' and 'pmic_version' elements take the form of
> >> > +v<Major>.<Minor> where the minor number may be omitted when it's zero, i.e.
> >> > +v1.0 is the same as v1. If all versions of the 'plat_version' element's match,
> >> > +then a wildcard '*' should be used, e.g. 'v*'.
> >> > +
> >> > +The 'foundry_id', 'subtype', and 'mb' elements are one or more digits from 0
> >> > +to 9.
> >>
> >> Can you define what these are exactly. I gather mb is RAM size.
> >
> > Not really, foundry_id is a number and so is subtype.
> 
> For mb, can't the tool just parse the memory node to get ram size
> rather than parsing the compatible.

Sure. Right now the bootloader injects the memory information
during boot. I think it should work if we already have the memory
information there. I don't have any usage of mb at the moment
though, so if you want we can drop this field until a time that
we need it.

> 
> >> > +
> >> > +The 'panel' element must be one of the following strings:
> >> > +
> >> > +   720p
> >> > +   fWVGA
> >> > +   hd
> >> > +   qHD
> >>
> >> How is this used?
> >
> > I believe this was added so that we could have different dtbs for
> > devices that have different panels on them. I'm not sure this is
> > still used though. It could be legacy.
> 
> Dealing with multiple panels is fairly common. I think you could use
> an alias to the panel node and match using its compatible or
> resolution.

So dtbtool will need to resolve the alias and then figure out
which type of panel it is from compatible? Ok. I'd rather not
write that code unless it's needed, so I hope this field is not
used either.

> 
> >> > +The 'boot' element must be one of the following strings:
> >> > +
> >> > +   emmc_sdc1
> >> > +   ufs
> >> > +
> 
> Again, perhaps an alias would work here.
> 
> >> > +The 'pmic' element must be one of the following strings:
> >> > +
> >> > +   pm8841
> >> > +   pm8019
> >> > +   pm8110
> >> > +   pma8084
> >> > +   pmi8962
> >> > +   pmd9635
> >> > +   pm8994
> >> > +   pmi8994
> >> > +   pm8916
> >> > +   pm8004
> >> > +   pm8909
> >> > +
> >> > +The 'pmic' element is specified in order of ascending USID. The PMIC in USID0
> >> > +goes first, and then USID2, USID4, and finally USID6. Up to four PMICs may be
> >> > +specified and no holes in the USID number space are allowed.
> >>
> >> What is USID?
> >
> > USID is Unique Slave IDentifier. It's an SPMI concept.
> 
> Okay, then please say "SPMI USID" or something.

Ok.

In attempts to shorten the compatible string, we could use
aliases for the USIDs too. Then it should be possible to pull out
the information from the compatible fields of the PMIC nodes to
construct the PMIC part of the string. I'd like to avoid having
to go down the path of / -> soc -> spmi controller -> usidx,y,z
and just go straight to the usid node from a phandle.

With that in mind, right now I'm using fdtget from python to grab
the compatible string and parse it with a regex. If things need
to become more complicated to start following phandles, etc. are
there some python bindings to libfdt somewhere?

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