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

[Richard Fitzgerald <rf@opensource.cirrus.com>]

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@opensource.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@opensource.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.
> 

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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