Patchwork pwm: Fix period and polarity in pwm_get() for non-perfect matches

login
register
mail settings
Submitter Geert Uytterhoeven
Date Aug. 13, 2014, 3:18 p.m.
Message ID <1407943133-18170-1-git-send-email-geert+renesas@glider.be>
Download mbox | patch
Permalink /patch/379672/
State Accepted
Headers show

Comments

Geert Uytterhoeven - Aug. 13, 2014, 3:18 p.m.
If pwm_get() finds a look-up entry with a perfect match (both dev_id and
con_id match), the loop is aborted, and "p" still points to the correct
struct pwm_lookup.

If only an entry with a matching dev_id or con_id is found, the loop
terminates after traversing the whole list, and "p" now points to
arbitrary memory, not part of the pwm_lookup list.
Then pwm_set_period() and pwm_set_polarity() will set random values for
period resp. polarity.

To fix this, save period and polarity when finding a new best match,
just like is done for chip (for the provider) and index.

This fixes the LCD backlight on r8a7740/armadillo-legacy, which was fed
period 0 and polarity -1068821144 instead of 33333 resp. 1.

Fixes: 3796ce1d4d4b330a75005c5eda105603ce9d4071 ("pwm: add period and polarity to struct pwm_lookup")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: stable@vger.kernel.org
---
 drivers/pwm/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
Thierry Reding - Aug. 18, 2014, 8:20 a.m.
On Wed, Aug 13, 2014 at 05:18:53PM +0200, Geert Uytterhoeven wrote:
> If pwm_get() finds a look-up entry with a perfect match (both dev_id and
> con_id match), the loop is aborted, and "p" still points to the correct
> struct pwm_lookup.
> 
> If only an entry with a matching dev_id or con_id is found, the loop
> terminates after traversing the whole list, and "p" now points to
> arbitrary memory, not part of the pwm_lookup list.
> Then pwm_set_period() and pwm_set_polarity() will set random values for
> period resp. polarity.
> 
> To fix this, save period and polarity when finding a new best match,
> just like is done for chip (for the provider) and index.
> 
> This fixes the LCD backlight on r8a7740/armadillo-legacy, which was fed
> period 0 and polarity -1068821144 instead of 33333 resp. 1.
> 
> Fixes: 3796ce1d4d4b330a75005c5eda105603ce9d4071 ("pwm: add period and polarity to struct pwm_lookup")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> Cc: stable@vger.kernel.org
> ---
>  drivers/pwm/core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)

Good catch! One comment below.

> diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> index 4b66bf09ee55..d2c35920ff08 100644
> --- a/drivers/pwm/core.c
> +++ b/drivers/pwm/core.c
> @@ -606,6 +606,8 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
>  	unsigned int best = 0;
>  	struct pwm_lookup *p;
>  	unsigned int match;
> +	unsigned int period;
> +	enum pwm_polarity polarity;
>  
>  	/* look up via DT first */
>  	if (IS_ENABLED(CONFIG_OF) && dev && dev->of_node)
> @@ -653,6 +655,8 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
>  		if (match > best) {
>  			chip = pwmchip_find_by_name(p->provider);
>  			index = p->index;
> +			period = p->period;
> +			polarity = p->polarity;
>  
>  			if (match != 3)
>  				best = match;
> @@ -668,8 +672,8 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
>  	if (IS_ERR(pwm))
>  		return pwm;
>  
> -	pwm_set_period(pwm, p->period);
> -	pwm_set_polarity(pwm, p->polarity);
> +	pwm_set_period(pwm, period);
> +	pwm_set_polarity(pwm, polarity);

Could we achieve the same by storing a pointer to the best match and
then use that instead of p? Perhaps something like this:

	struct pwm_lookup *entry;

	...

		if (match > best) {
			chip = pwmchip_find_by_name(p->provider);
			entry = p;

			if (match != 3)
				best = match;
			else
				break;
		}

	...

	if (chip)
		pwm = pwm_request_from_chip(chip, entry->index,
					    con_id ?: dev_id);
	if (IS_ERR(pwm))
		return pwm;

	pwm_set_period(pwm, entry->period);
	pwm_set_polarity(pwm, entry->polarity);

?

Thierry
Geert Uytterhoeven - Aug. 18, 2014, 8:38 a.m.
Hi Thierry,

On Mon, Aug 18, 2014 at 10:20 AM, Thierry Reding
<thierry.reding@gmail.com> wrote:
> Could we achieve the same by storing a pointer to the best match and
> then use that instead of p? Perhaps something like this:
>
>         struct pwm_lookup *entry;
>
>         ...
>
>                 if (match > best) {
>                         chip = pwmchip_find_by_name(p->provider);
>                         entry = p;
>
>                         if (match != 3)
>                                 best = match;
>                         else
>                                 break;
>                 }
>
>         ...
>
>         if (chip)
>                 pwm = pwm_request_from_chip(chip, entry->index,
>                                             con_id ?: dev_id);
>         if (IS_ERR(pwm))
>                 return pwm;
>
>         pwm_set_period(pwm, entry->period);
>         pwm_set_polarity(pwm, entry->polarity);
>
> ?

That's possible. But that will add complexity, as you have to move the
"mutex_unlock(&pwm_lookup_lock);" after the last user of "entry" again,
and add a goto for the IS_ERR(pwm) case.
So I'm not sure it's worth the effort.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
--
To unsubscribe from this list: send the line "unsubscribe linux-pwm" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Thierry Reding - Aug. 18, 2014, 8:57 a.m.
On Mon, Aug 18, 2014 at 10:38:00AM +0200, Geert Uytterhoeven wrote:
> Hi Thierry,
> 
> On Mon, Aug 18, 2014 at 10:20 AM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> > Could we achieve the same by storing a pointer to the best match and
> > then use that instead of p? Perhaps something like this:
> >
> >         struct pwm_lookup *entry;
> >
> >         ...
> >
> >                 if (match > best) {
> >                         chip = pwmchip_find_by_name(p->provider);
> >                         entry = p;
> >
> >                         if (match != 3)
> >                                 best = match;
> >                         else
> >                                 break;
> >                 }
> >
> >         ...
> >
> >         if (chip)
> >                 pwm = pwm_request_from_chip(chip, entry->index,
> >                                             con_id ?: dev_id);
> >         if (IS_ERR(pwm))
> >                 return pwm;
> >
> >         pwm_set_period(pwm, entry->period);
> >         pwm_set_polarity(pwm, entry->polarity);
> >
> > ?
> 
> That's possible. But that will add complexity, as you have to move the
> "mutex_unlock(&pwm_lookup_lock);" after the last user of "entry" again,
> and add a goto for the IS_ERR(pwm) case.
> So I'm not sure it's worth the effort.

Oh, right. It would've been nice to avoid all the temporary variables,
but we can always refactor if that turns out to become too messy. I'll
apply this one for now.

I'll shorten the SHA-1 in the Fixes: line to 12 characters if you don't
mind to match the guidelines in Documentation/SubmittingPatches.

Thierry

Patch

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index 4b66bf09ee55..d2c35920ff08 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -606,6 +606,8 @@  struct pwm_device *pwm_get(struct device *dev, const char *con_id)
 	unsigned int best = 0;
 	struct pwm_lookup *p;
 	unsigned int match;
+	unsigned int period;
+	enum pwm_polarity polarity;
 
 	/* look up via DT first */
 	if (IS_ENABLED(CONFIG_OF) && dev && dev->of_node)
@@ -653,6 +655,8 @@  struct pwm_device *pwm_get(struct device *dev, const char *con_id)
 		if (match > best) {
 			chip = pwmchip_find_by_name(p->provider);
 			index = p->index;
+			period = p->period;
+			polarity = p->polarity;
 
 			if (match != 3)
 				best = match;
@@ -668,8 +672,8 @@  struct pwm_device *pwm_get(struct device *dev, const char *con_id)
 	if (IS_ERR(pwm))
 		return pwm;
 
-	pwm_set_period(pwm, p->period);
-	pwm_set_polarity(pwm, p->polarity);
+	pwm_set_period(pwm, period);
+	pwm_set_polarity(pwm, polarity);
 
 
 	return pwm;