linux-kernel - Re: [PATCH 1/7] of: base: Add of_count_phandle_with_fixed

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Date:   Fri, 16 Oct 2020 08:31:47 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Robin Murphy <robin.murphy@....com>,
        Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc:     devicetree@...r.kernel.org,
        Linux-ALSA <alsa-devel@...a-project.org>,
        - <patches@...nsource.cirrus.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        "moderated list:BROADCOM BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/7] of: base: Add of_count_phandle_with_fixed_args()

On Thu, Oct 15, 2020 at 11:52 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2020-10-14 19:39, Rob Herring wrote:
> > On Wed, Oct 14, 2020 at 9:54 AM Richard Fitzgerald
> > <rf@...nsource.cirrus.com> 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@...nsource.cirrus.com>
> >> ---
> >>   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.
>
> >> +       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 :/

Indeed. We should just generalize this. It can still be refactored to
shared code.

It's probably worthwhile to check for a remainder here IMO.

Rob