[RFC] of: Allow for experimental device tree bindings

[jonsmirl at gmail.com]

On Wed, Oct 23, 2013 at 11:06 AM, Thierry Reding
<thierry.reding at gmail.com> wrote:
> Past and recent discussions have shown that there's some concensus that
> device tree bindings should be considered an ABI and therefore need to
> remain backwards-compatible once code to implement them is included in
> the final release of a Linux kernel.

Doing it this way clutters up the DTS source files.

Schemas are a way to do this without clutter. There should be a master
schema that would validate all accepted device tree bindings.   So
experimental bindings naturally drop right of of this - they'll
generate errors when validated against the master schema.  When the
binding is finally sorted out it gets added to the master schema and
the validation errors go away.


>
> At the same time there is a desire to keep some manoeuvre while we're
> trying to figure things out. The fact is that many of us lack the
> experience to design good bindings from the start. At the same time
> there is a shortage of people that can review bindings and help design
> better ones.
>
> Progress and the addition of new features (and restoration of features
> that used to work before the advent of DT for that matter) are blocked
> to a large degree because of that.
>
> This patch attempts to restore some degree of freedom by introducing an
> easy way to mark device tree bindings as experimental as well as a way
> for users to disable the use of experimental bindings if they choose
> functionality at the cost of potential device tree incompatibilities.
>
> Bindings are marked experimental by prefixing the compatible value with
> an exclamation mark (!). In order to make it clear that experimental
> bindings are undesirable in the long run, a warning will be output when
> an experimental binding is encountered.
>
> Signed-off-by: Thierry Reding <treding at nvidia.com>
> ---
>  drivers/of/Kconfig |  7 +++++++
>  drivers/of/base.c  | 32 +++++++++++++++++++++++++++++++-
>  2 files changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index 78cc760..dc482f8 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -24,6 +24,13 @@ config OF_SELFTEST
>
>           If unsure, say N here, but this option is safe to enable.
>
> +config OF_EXPERIMENTAL
> +       bool "Support experimental device tree bindings"
> +       help
> +         This option allows experimental device tree bindings to be used.
> +         Note that experimental bindings are subject to change, possibly
> +         requiring the DTB to be updated.
> +
>  config OF_FLATTREE
>         bool
>         select DTC
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index a96f850..b0b8371 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -342,6 +342,36 @@ struct device_node *of_get_cpu_node(int cpu, unsigned int *thread)
>  }
>  EXPORT_SYMBOL(of_get_cpu_node);
>
> +static int of_compat_match(const char *device, const char *driver,
> +                          const struct device_node *np)
> +{
> +       const char *dev = device;
> +       const char *drv = driver;
> +
> +       if (device[0] == '!')
> +               device++;
> +
> +       if (driver[0] == '!')
> +               driver++;
> +
> +       if (of_compat_cmp(device, driver, strlen(driver)) != 0)
> +               return 0;
> +
> +       /* check if binding is experimental */
> +       if (dev != device || drv != driver) {
> +               pr_warn("of: device %s (%s) uses an experimental binding\n",
> +                       np->name, np->full_name);
> +
> +               /* don't match if we don't want experimental bindings */
> +               if (!IS_ENABLED(CONFIG_OF_EXPERIMENTAL)) {
> +                       pr_err("of: refusing to use binding \"%s\"\n", device);
> +                       return 0;
> +               }
> +       }
> +
> +       return 1;
> +}
> +
>  /** Checks if the given "compat" string matches one of the strings in
>   * the device's "compatible" property
>   */
> @@ -355,7 +385,7 @@ static int __of_device_is_compatible(const struct device_node *device,
>         if (cp == NULL)
>                 return 0;
>         while (cplen > 0) {
> -               if (of_compat_cmp(cp, compat, strlen(compat)) == 0)
> +               if (of_compat_match(cp, compat, device))
>                         return 1;
>                 l = strlen(cp) + 1;
>                 cp += l;
> --
> 1.8.4
>
>
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-- 
Jon Smirl
jonsmirl at gmail.com