Re: [PATCH 1/7] of: base: Add of_count_phandle_with_fixed_args()

[Richard Fitzgerald]

On 15/10/2020 17:52, Robin Murphy wrote:
> On 2020-10-14 19:39, Rob Herring wrote:
> > On Wed, Oct 14, 2020 at 9:54 AM Richard Fitzgerald
> > <rf@xxxxxxxxxxxxxxxxxxxxx> wrote:
> > > Add an equivalent of of_count_phandle_with_args() for fixed argument
> > > sets, to pair with of_parse_phandle_with_fixed_args().
> > >
> > > Signed-off-by: Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >   drivers/of/base.c  | 42 ++++++++++++++++++++++++++++++++++++++++++
> > >   include/linux/of.h |  9 +++++++++
> > >   2 files changed, 51 insertions(+)
> > >
> > > diff --git a/drivers/of/base.c b/drivers/of/base.c
> > > index ea44fea99813..45d8b0e65345 100644
> > > --- a/drivers/of/base.c
> > > +++ b/drivers/of/base.c
> > > @@ -1772,6 +1772,48 @@ int of_count_phandle_with_args(const struct 
> > > device_node *np, const char *list_na
> > >   }
> > >   EXPORT_SYMBOL(of_count_phandle_with_args);
> > >
> > > +/**
> > > + * of_count_phandle_with_fixed_args() - Find the number of phandles 
> > > references in a property
> > > + * @np:                pointer to a device tree node containing a list
> > > + * @list_name: property name that contains a list
> > > + * @cell_count: number of argument cells following the phandle
> > > + *
> > > + * Returns the number of phandle + argument tuples within a 
> > > property. It
> > > + * is a typical pattern to encode a list of phandle and variable
> > > + * arguments into a single property.
> > > + */
> > > +int of_count_phandle_with_fixed_args(const struct device_node *np,
> > > +                                    const char *list_name,
> > > +                                    int cells_count)
> > > +{
> >
> > Looks to me like you can refactor of_count_phandle_with_args to handle
> > both case and then make this and of_count_phandle_with_args simple
> > wrapper functions.
>
> Although for just counting the number of phandles each with n arguments 
> that a property contains, isn't that simply a case of dividing the 
> property length by n + 1? The phandles themselves will be validated by 
> any subsequent of_parse_phandle*() call anyway, so there doesn't seem 
> much point in doing more work then necessary here.

As I'm not a DT expert, I'm reluctant to change existing algorithms that
could break everything just for the trivial case of adding a fixed
arguments count. I have a re-worked patch as suggested by Rob that
re-uses the existing counting function for both cases.

> > > +       struct of_phandle_iterator it;
> > > +       int rc, cur_index = 0;
> > > +
> > > +       if (!cells_count) {
> > > +               const __be32 *list;
> > > +               int size;
> > > +
> > > +               list = of_get_property(np, list_name, &size);
> > > +               if (!list)
> > > +                       return -ENOENT;
> > > +
> > > +               return size / sizeof(*list);
>
> Case in point - if it's OK to do exactly that for n == 0, then clearly 
> we're *aren't* fussed about validating anything, so the n > 0 code below 
> is nothing more than a massively expensive way to check for a nonzero 
> remainder :/
>
> Robin.
>
> > > +       }
> > > +
> > > +       rc = of_phandle_iterator_init(&it, np, list_name, NULL, 
> > > cells_count);
> > > +       if (rc)
> > > +               return rc;
> > > +
> > > +       while ((rc = of_phandle_iterator_next(&it)) == 0)
> > > +               cur_index += 1;
> > > +
> > > +       if (rc != -ENOENT)
> > > +               return rc;
> > > +
> > > +       return cur_index;
> > > +}
> > > +EXPORT_SYMBOL(of_count_phandle_with_fixed_args);
> > > +
> > >   /**
> > >    * __of_add_property - Add a property to a node without lock 
> > > operations
> > >    */
> > > diff --git a/include/linux/of.h b/include/linux/of.h
> > > index 5cf7ae0465d1..9f315da4e9da 100644
> > > --- a/include/linux/of.h
> > > +++ b/include/linux/of.h
> > > @@ -377,6 +377,8 @@ extern int of_parse_phandle_with_fixed_args(const 
> > > struct device_node *np,
> > >          struct of_phandle_args *out_args);
> > >   extern int of_count_phandle_with_args(const struct device_node *np,
> > >          const char *list_name, const char *cells_name);
> > > +extern int of_count_phandle_with_fixed_args(const struct device_node 
> > > *np,
> > > +       const char *list_name, int cells_count);
> > >
> > >   /* phandle iterator functions */
> > >   extern int of_phandle_iterator_init(struct of_phandle_iterator *it,
> > > @@ -886,6 +888,13 @@ static inline int 
> > > of_count_phandle_with_args(struct device_node *np,
> > >          return -ENOSYS;
> > >   }
> > >
> > > +static inline int of_count_phandle_with_fixed_args(const struct 
> > > device_node *np,
> > > +                                                  const char 
> > > *list_name,
> > > +                                                  int cells_count)
> > > +{
> > > +       return -ENOSYS;
> > > +}
> > > +
> > >   static inline int of_phandle_iterator_init(struct 
> > > of_phandle_iterator *it,
> > >                                             const struct device_node 
> > > *np,
> > >                                             const char *list_name,
> > > --
> > > 2.20.1
> >
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