Re: [PATCHv5 2/3] pwm: make the PWM_POLARITY flag in DTB optional

[Thierry Reding]

On Tue, Oct 07, 2014 at 03:55:33PM +0200, Lothar Waßmann wrote:
> Change the pwm chip driver registration, so that a chip driver that
> supports polarity inversion can still be used with DTBs that don't
> provide the 'PWM_POLARITY' flag.
> 
> This is done to provide polarity inversion support for the pwm-imx
> driver without having to modify all existing DTS files.

I don't like how this throws out the window the only sanity checking we
have in place for the #pwm-cells property.

As I understand it, the problem that you're trying to solve is one of
backwards-compatibility where existing device trees have #pwm-cells =
<2>, but the driver is extended to support flags as well.

In that case, can we not simply make of_pwm_xlate_with_flags() support
that case transparently? That is, if the driver sets .of_pwm_n_cells to
3, we can still support #pwm-cells = <2> and use the default (no) flags
instead.

Something like the below should do that (compile-tested only).

Thierry

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index 966497d10c6e..89a5e309b0a3 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -136,9 +136,14 @@ of_pwm_xlate_with_flags(struct pwm_chip *pc, const struct 
of_phandle_args *args)
 {
        struct pwm_device *pwm;
 
+       /* check that the driver supports a third cell for flags */
        if (pc->of_pwm_n_cells < 3)
                return ERR_PTR(-EINVAL);
 
+       /* flags in the third cell are optional */
+       if (args->args_count < 2)
+               return ERR_PTR(-EINVAL);
+
        if (args->args[0] >= pc->npwm)
                return ERR_PTR(-EINVAL);
 
@@ -148,10 +153,12 @@ of_pwm_xlate_with_flags(struct pwm_chip *pc, const struct 
of_phandle_args *args)
 
        pwm_set_period(pwm, args->args[1]);
 
-       if (args->args[2] & PWM_POLARITY_INVERTED)
-               pwm_set_polarity(pwm, PWM_POLARITY_INVERSED);
-       else
-               pwm_set_polarity(pwm, PWM_POLARITY_NORMAL);
+       if (args->args_count > 2) {
+               if (args->args[2] & PWM_POLARITY_INVERTED)
+                       pwm_set_polarity(pwm, PWM_POLARITY_INVERSED);
+               else
+                       pwm_set_polarity(pwm, PWM_POLARITY_NORMAL);
+       }
 
        return pwm;
 }
@@ -162,9 +169,14 @@ of_pwm_simple_xlate(struct pwm_chip *pc, const struct 
of_phandle_args *args)
 {
        struct pwm_device *pwm;
 
+       /* sanity check driver support */
        if (pc->of_pwm_n_cells < 2)
                return ERR_PTR(-EINVAL);
 
+       /* all cells are required */
+       if (args->args_count != pc->of_pwm_n_cells)
+               return ERR_PTR(-EINVAL);
+
        if (args->args[0] >= pc->npwm)
                return ERR_PTR(-EINVAL);
 
@@ -536,13 +548,6 @@ struct pwm_device *of_pwm_get(struct device_node *np, 
const char *con_id)
                goto put;
        }
 
-       if (args.args_count != pc->of_pwm_n_cells) {
-               pr_debug("%s: wrong #pwm-cells for %s\n", np->full_name,
-                        args.np->full_name);
-               pwm = ERR_PTR(-EINVAL);
-               goto put;
-       }
-
        pwm = pc->of_xlate(pc, &args);
        if (IS_ERR(pwm))
                goto put;

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