Re: [PATCH 01/23] fdt: Add functions to query a node's #address- and #size-cells

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Fri, Aug 22, 2014 at 09:03:49PM -0600, Simon Glass wrote:
> Hi Thierry,
> 
> On 19 August 2014 07:06, Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > On Tue, Aug 19, 2014 at 06:52:22AM -0600, Simon Glass wrote:
> >> Hi Theirry,
> >>
> >>
> >> On 19 August 2014 04:59, Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >>
> >> > On Mon, Aug 18, 2014 at 11:52:36AM -0600, Simon Glass wrote:
> >> > > Hi Thierry,
> >> > >
> >> > > On 18 August 2014 01:16, Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> >> > wrote:
> >> > > > From: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >> > > >
> >> > > > Given a device tree node, the fdt_n_addr_cells() function will walk up
> >> > > > the device tree and search for an #address-cells property. It returns
> >> > > > the number of cells required by the device tree node to represent an
> >> > > > address.
> >> > > >
> >> > > > Similarly the fdt_n_size_cells() function returns the number of cells
> >> > > > required by the given device tree node to represent a size. It achieves
> >> > > > that by walking up the device tree in seach for a #size-cells property.
> >> > > >
> >> > > > If no #address-cells or #size-cells property can be found, both of the
> >> > > > functions return 1.
> >> > > >
> >> > > > Signed-off-by: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >> > > > ---
> >> > > >  include/libfdt.h    | 28 ++++++++++++++++++++++++++++
> >> > > >  lib/libfdt/fdt_ro.c | 36 ++++++++++++++++++++++++++++++++++++
> >> > > >  2 files changed, 64 insertions(+)
> >> > > >
> >> > > > diff --git a/include/libfdt.h b/include/libfdt.h
> >> > > > index a1ef1e15df3d..e7f991b388cf 100644
> >> > > > --- a/include/libfdt.h
> >> > > > +++ b/include/libfdt.h
> >> > > > @@ -1638,4 +1638,32 @@ int fdt_find_regions(const void *fdt, char *
> >> > const inc[], int inc_count,
> >> > > >                      struct fdt_region region[], int max_regions,
> >> > > >                      char *path, int path_len, int add_string_tab);
> >> > > >
> >> > > > +/**
> >> > > > + * fdt_n_addr_cells() - find the number of address cells required by
> >> > a node
> >> > > > + *
> >> > > > + * Looks up the #address-cells property of the node to examine. If
> >> > that has
> >> > > > + * no such property, walks up the device tree until it finds one in
> >> > one of
> >> > > > + * the device's parents. If no #address-cells property is found, it is
> >> > > > + * assumed to be 1.
> >> > > > + *
> >> > > > + * @param fdt          FDT blob
> >> > > > + * @param node         node to examine
> >> > > > + * @return number of address cells
> >> > > > + */
> >> > > > +int fdt_n_addr_cells(const void *fdt, int node);
> >> > >
> >> > > There is a new fdt_address_cells() recently that looks suitable.
> >> > >
> >> > > > +
> >> > > > +/**
> >> > > > + * fdt_n_size_cells() - find the number of size cells required by a
> >> > node
> >> > >
> >> > > Also fdt_size_cells().
> >> >
> >> > Neither of those seem to walk up the tree, so inheriting #address-cells
> >> > or #size-cells from a parent will not work.
> >> >
> >> > According to the comments in the code they're also supposed to return
> >> > the number of cells represented by a bus, not a device (which might
> >> > explain why it doesn't walk up the tree).
> >> >
> >> > I also can't reuse them to implement these fdt_n_addr_cells() and
> >> > fdt_n_size_cells() functions because they don't return an accurate error
> >> > if the #address-cells and #size-cells are not found, therefore it's
> >> > impossible to decide when to climb further up the tree.
> >> >
> >>
> >> OK but I have one more question. I thought that dtc gave a warning when you
> >> don't explicitly specify these values in the parent node?
> >
> > It doesn't explicitly warn about the properties being absent. Rather it
> > will fallback to the default (#address-cells = <2>, #size-cells = <1> in
> > the version that I have) and warn if those don't match what's expected.
> >
> > Interestingly as opposed to what the Linux kernel does, DTC doesn't seem
> > to climb further up the tree if it can't find the properties in a device
> > tree node. Instead, it seems to always use the default values instead.
> >
> > Adding the devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org mailing list. Maybe somebody there
> > can help iron out this inconsistency. ePAPR explicitly says that both
> > "... #address-cells and #size-cells properties are not inherited from
> > ancestors in the device tree. They shall be explicitly defined." (see
> > 2.3.5 "#address-cells and #size-cells").
> 
> That's my understanding too. So should we drop this patch?

Yes, in this particular case it works fine using the existing
fdt_address_cells() and fdt_size_cells() functions so I've dropped this
patch.

Thierry

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