| Message ID | 217877237.2758643.1345731587941.JavaMail.root@advansee.com |
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On 08/23/2012 04:19 PM, Benoît Thébaudeau wrote: > Some PWM drivers enable the clock of the PWM peripheral in pwm_enable(). Hence, > for these drivers, a call to pwm_config() does not have any effect before > pwm_enable() has been called. > > This patch fixes the PWM users to make sure that they call pwm_enable() before > pwm_config(). > > This fixes the first setting of brightness through sysfs that had no effect with > leds-pwm and the i.MX PWM driver. But isn't this a bug in the PWM peripheral driver? With this change the PWM will start with the old settings first. While this is not so much of a problem for a backlight (although it might cause a short flickering) it might cause problems for other applications, like using the PWM pin as a timing generator. In my opinion it's better to fix the PWM peripheral drivers which have this problem instead of trying to work around it in every user of the PWM API. - Lars
On Thursday, August 23, 2012 5:43:32 PM, Lars-Peter Clausen wrote: > On 08/23/2012 04:19 PM, Benoît Thébaudeau wrote: > > Some PWM drivers enable the clock of the PWM peripheral in > > pwm_enable(). Hence, > > for these drivers, a call to pwm_config() does not have any effect > > before > > pwm_enable() has been called. > > > > This patch fixes the PWM users to make sure that they call > > pwm_enable() before > > pwm_config(). > > > > This fixes the first setting of brightness through sysfs that had > > no effect with > > leds-pwm and the i.MX PWM driver. > > But isn't this a bug in the PWM peripheral driver? With this change > the PWM > will start with the old settings first. While this is not so much of > a problem > for a backlight (although it might cause a short flickering) it might > cause > problems for other applications, like using the PWM pin as a timing > generator. > In my opinion it's better to fix the PWM peripheral drivers which > have this > problem instead of trying to work around it in every user of the PWM > API. I don't know. See my detailed description of this issue here: http://lists.infradead.org/pipermail/linux-arm-kernel/2012-August/115667.html Where the bug is depends on the detailed definition of the PWM API, which I don't find documented anywhere. If pwm_enable() means "start PWM timer with the configured settings", then the bug is in the drivers. If it means "enable the PWM peripheral so that we can work with it", then the bug is in the PWM users. I don't really have time to work on this, so I suggested this patch as a simple solution. Otherwise, it means reworking several PWM drivers for different hardware that is not available to everyone for testing. If we decide to only change the i.MX PWM driver, the fix would be: pwm_config() { save passed config in private data; if (pwm enabled) apply passed config; } pwm_enable() { if (!(pwm enabled)) { enable pwm ip clk; apply config from private data; } } If we fix only this driver, we must not forget that the same issue probably exists with several other PWM drivers. As I said in my bug description, the PWM users set the duty cycle to 0 before calling pwm_disable(), so fixing the PWM users should not be an issue, even in the timing generator use case you're talking about. I don't have a strong opinion about what should be fixed here. Best regards, Benoît
On 08/23/2012 06:57 PM, Benoît Thébaudeau wrote: > On Thursday, August 23, 2012 5:43:32 PM, Lars-Peter Clausen wrote: >> On 08/23/2012 04:19 PM, Benoît Thébaudeau wrote: >>> Some PWM drivers enable the clock of the PWM peripheral in >>> pwm_enable(). Hence, >>> for these drivers, a call to pwm_config() does not have any effect >>> before >>> pwm_enable() has been called. >>> >>> This patch fixes the PWM users to make sure that they call >>> pwm_enable() before >>> pwm_config(). >>> >>> This fixes the first setting of brightness through sysfs that had >>> no effect with >>> leds-pwm and the i.MX PWM driver. >> >> But isn't this a bug in the PWM peripheral driver? With this change >> the PWM >> will start with the old settings first. While this is not so much of >> a problem >> for a backlight (although it might cause a short flickering) it might >> cause >> problems for other applications, like using the PWM pin as a timing >> generator. >> In my opinion it's better to fix the PWM peripheral drivers which >> have this >> problem instead of trying to work around it in every user of the PWM >> API. > > I don't know. See my detailed description of this issue here: > http://lists.infradead.org/pipermail/linux-arm-kernel/2012-August/115667.html > > Where the bug is depends on the detailed definition of the PWM API, which I > don't find documented anywhere. > > If pwm_enable() means "start PWM timer with the configured settings", then the > bug is in the drivers. If it means "enable the PWM peripheral so that we can > work with it", then the bug is in the PWM users. It really is the former. See the description of pwm_enable() in drivers/pwm/core.c > > I don't really have time to work on this, so I suggested this patch as a simple > solution. Otherwise, it means reworking several PWM drivers for different > hardware that is not available to everyone for testing. > > If we decide to only change the i.MX PWM driver, the fix would be: > pwm_config() > { > save passed config in private data; > if (pwm enabled) > apply passed config; > } > > pwm_enable() > { > if (!(pwm enabled)) { > enable pwm ip clk; > apply config from private data; > } > } Another option is to enable the clock if it is disabled when the device is configured. E.g. that's what tegra does. > > If we fix only this driver, we must not forget that the same issue probably > exists with several other PWM drivers. > Since this seems to be a common pattern in a number of PWM drivers it might make sense to simply add support for enabling/disabling a clk to the pwm core. Or maybe just use the runtime pm API for this. This probably makes even more sense and grab a reference to the pm context when the enable() is called, release it when disable() is called and also grab it before calling the device's config callback and release it afterward. - Lars
On 08/23/2012 07:12 PM, Lars-Peter Clausen wrote: >[...] > Or maybe just use the runtime pm API for this. This probably makes even more > sense and grab a reference to the pm context when the enable() is called, > release it when disable() is called and also grab it before calling the > device's config callback and release it afterward. Btw. this seems to be exactly what the pwm-tiecap and pwm-tiehrpwm drivers already do today. I'd just make that a core PWM framework feature.
On Thu, Aug 23, 2012 at 07:19:36PM +0200, Lars-Peter Clausen wrote: > On 08/23/2012 07:12 PM, Lars-Peter Clausen wrote: > >[...] > > Or maybe just use the runtime pm API for this. This probably makes even more > > sense and grab a reference to the pm context when the enable() is called, > > release it when disable() is called and also grab it before calling the > > device's config callback and release it afterward. > > Btw. this seems to be exactly what the pwm-tiecap and pwm-tiehrpwm drivers > already do today. I'd just make that a core PWM framework feature. Using runtime PM for this sounds indeed like the most generic approach. I'm not very familiar with the API, but I thought it required explicit architecture or bus support (or the driver itself can provide hooks via pm_ops), so it might be difficult to implement for platforms that don't have working runtime PM support. I'll have to look into this some more. Anyway, if we add this kind of support to the PWM core we should take it through next first. For drivers that are currently broken, they should be, as you said, fixed by enabling the clock before reconfiguring and disable it after (unless the PWM is enabled) like Tegra does. Thierry
On Thu, Aug 23, 2012 at 07:12:04PM +0200, Lars-Peter Clausen wrote: > On 08/23/2012 06:57 PM, Benoît Thébaudeau wrote: > > On Thursday, August 23, 2012 5:43:32 PM, Lars-Peter Clausen wrote: > >> On 08/23/2012 04:19 PM, Benoît Thébaudeau wrote: > >>> Some PWM drivers enable the clock of the PWM peripheral in > >>> pwm_enable(). Hence, > >>> for these drivers, a call to pwm_config() does not have any effect > >>> before > >>> pwm_enable() has been called. > >>> > >>> This patch fixes the PWM users to make sure that they call > >>> pwm_enable() before > >>> pwm_config(). > >>> > >>> This fixes the first setting of brightness through sysfs that had > >>> no effect with > >>> leds-pwm and the i.MX PWM driver. > >> > >> But isn't this a bug in the PWM peripheral driver? With this change > >> the PWM > >> will start with the old settings first. While this is not so much of > >> a problem > >> for a backlight (although it might cause a short flickering) it might > >> cause > >> problems for other applications, like using the PWM pin as a timing > >> generator. > >> In my opinion it's better to fix the PWM peripheral drivers which > >> have this > >> problem instead of trying to work around it in every user of the PWM > >> API. > > > > I don't know. See my detailed description of this issue here: > > http://lists.infradead.org/pipermail/linux-arm-kernel/2012-August/115667.html > > > > Where the bug is depends on the detailed definition of the PWM API, which I > > don't find documented anywhere. > > > > If pwm_enable() means "start PWM timer with the configured settings", then the > > bug is in the drivers. If it means "enable the PWM peripheral so that we can > > work with it", then the bug is in the PWM users. > > It really is the former. See the description of pwm_enable() in drivers/pwm/core.c Yes. pwm_enable() is only for starting the PWM and *not* the peripheral. I should update the documentation to make this clearer. > > I don't really have time to work on this, so I suggested this patch as a simple > > solution. Otherwise, it means reworking several PWM drivers for different > > hardware that is not available to everyone for testing. > > > > If we decide to only change the i.MX PWM driver, the fix would be: > > pwm_config() > > { > > save passed config in private data; > > if (pwm enabled) > > apply passed config; > > } > > > > pwm_enable() > > { > > if (!(pwm enabled)) { > > enable pwm ip clk; > > apply config from private data; > > } > > } > > Another option is to enable the clock if it is disabled when the device is > configured. E.g. that's what tegra does. Yes, that would have been my proposal as well. > > If we fix only this driver, we must not forget that the same issue probably > > exists with several other PWM drivers. > > Since this seems to be a common pattern in a number of PWM drivers it might > make sense to simply add support for enabling/disabling a clk to the pwm core. > Or maybe just use the runtime pm API for this. This probably makes even more > sense and grab a reference to the pm context when the enable() is called, > release it when disable() is called and also grab it before calling the > device's config callback and release it afterward. The problem with adding this kind of support in the core is that the device or platform doesn't necessarily support runtime PM. Perhaps adding a flag to mark compatible drivers would be an option. If the runtime PM API gracefully handles cases where no callbacks are implemented (and therefore assumed unneeded) things should also work. On the other hand this really is very driver-specific stuff. A lot of hardware doesn't require an explicit clock being enabled to write registers so those wouldn't need the implicit PM calls either. Thierry
On Friday, August 24, 2012 12:44 AM, Lars-Peter Clausen wrote: > > On 08/23/2012 04:19 PM, Benoît Thébaudeau wrote: > > Some PWM drivers enable the clock of the PWM peripheral in pwm_enable(). Hence, > > for these drivers, a call to pwm_config() does not have any effect before > > pwm_enable() has been called. > > > > This patch fixes the PWM users to make sure that they call pwm_enable() before > > pwm_config(). > > > > This fixes the first setting of brightness through sysfs that had no effect with > > leds-pwm and the i.MX PWM driver. > > But isn't this a bug in the PWM peripheral driver? With this change the PWM > will start with the old settings first. While this is not so much of a problem > for a backlight (although it might cause a short flickering) it might cause > problems for other applications, like using the PWM pin as a timing generator. > In my opinion it's better to fix the PWM peripheral drivers which have this > problem instead of trying to work around it in every user of the PWM API. Hi Lars-Peter Clausen, I think so. It would be better to fix the PWM peripheral drivers. Best regards, Jingoo Han > > - Lars > -- > To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Aug 23, 2012 at 07:12:04PM +0200, Lars-Peter Clausen wrote: > On 08/23/2012 06:57 PM, Beno??t Th??baudeau wrote: > > exists with several other PWM drivers. > Since this seems to be a common pattern in a number of PWM drivers it might > make sense to simply add support for enabling/disabling a clk to the pwm core. > Or maybe just use the runtime pm API for this. This probably makes even more > sense and grab a reference to the pm context when the enable() is called, > release it when disable() is called and also grab it before calling the > device's config callback and release it afterward. It's a problem with IPs in general, a good proportion of modern SoCs need a clock supplying in order to interact with the registers on most of their IPs. runtime PM is the normal way forwards, Tegra recently started using regmap's cache code to handle this transparently in the driver.
On Thu, Aug 23, 2012 at 09:04:02PM +0200, Thierry Reding wrote: > Using runtime PM for this sounds indeed like the most generic approach. > I'm not very familiar with the API, but I thought it required explicit > architecture or bus support (or the driver itself can provide hooks via > pm_ops), so it might be difficult to implement for platforms that don't > have working runtime PM support. I'll have to look into this some more. It doesn't require explicit arch support, though many architectures choose to integrate their PM frameworks with it either directly or via power domains. If the architecture doesn't do anything with it then it should just work at the driver level.
diff --git linux-next-c94456b.orig/drivers/input/misc/pwm-beeper.c linux-next-c94456b/drivers/input/misc/pwm-beeper.c index fc84c8a..97d322b 100644 --- linux-next-c94456b.orig/drivers/input/misc/pwm-beeper.c +++ linux-next-c94456b/drivers/input/misc/pwm-beeper.c @@ -53,10 +53,10 @@ static int pwm_beeper_event(struct input_dev *input, pwm_disable(beeper->pwm); } else { period = HZ_TO_NANOSECONDS(value); - ret = pwm_config(beeper->pwm, period / 2, period); + ret = pwm_enable(beeper->pwm); if (ret) return ret; - ret = pwm_enable(beeper->pwm); + ret = pwm_config(beeper->pwm, period / 2, period); if (ret) return ret; beeper->period = period; @@ -156,8 +156,8 @@ static int pwm_beeper_resume(struct device *dev) struct pwm_beeper *beeper = dev_get_drvdata(dev); if (beeper->period) { - pwm_config(beeper->pwm, beeper->period / 2, beeper->period); pwm_enable(beeper->pwm); + pwm_config(beeper->pwm, beeper->period / 2, beeper->period); } return 0; diff --git linux-next-c94456b.orig/drivers/leds/leds-pwm.c linux-next-c94456b/drivers/leds/leds-pwm.c index f2e44c7..c2e0c22 100644 --- linux-next-c94456b.orig/drivers/leds/leds-pwm.c +++ linux-next-c94456b/drivers/leds/leds-pwm.c @@ -42,8 +42,8 @@ static void led_pwm_set(struct led_classdev *led_cdev, pwm_config(led_dat->pwm, 0, period); pwm_disable(led_dat->pwm); } else { - pwm_config(led_dat->pwm, brightness * period / max, period); pwm_enable(led_dat->pwm); + pwm_config(led_dat->pwm, brightness * period / max, period); } } diff --git linux-next-c94456b.orig/drivers/video/backlight/pwm_bl.c linux-next-c94456b/drivers/video/backlight/pwm_bl.c index 995f016..a4bb95c 100644 --- linux-next-c94456b.orig/drivers/video/backlight/pwm_bl.c +++ linux-next-c94456b/drivers/video/backlight/pwm_bl.c @@ -65,8 +65,8 @@ static int pwm_backlight_update_status(struct backlight_device *bl) duty_cycle = pb->lth_brightness + (duty_cycle * (pb->period - pb->lth_brightness) / max); - pwm_config(pb->pwm, duty_cycle, pb->period); pwm_enable(pb->pwm); + pwm_config(pb->pwm, duty_cycle, pb->period); } if (pb->notify_after)
Some PWM drivers enable the clock of the PWM peripheral in pwm_enable(). Hence, for these drivers, a call to pwm_config() does not have any effect before pwm_enable() has been called. This patch fixes the PWM users to make sure that they call pwm_enable() before pwm_config(). This fixes the first setting of brightness through sysfs that had no effect with leds-pwm and the i.MX PWM driver. Cc: Thierry Reding <thierry.reding@avionic-design.de> Cc: <linux-kernel@vger.kernel.org> Cc: Sascha Hauer <kernel@pengutronix.de> Cc: <linux-arm-kernel@lists.infradead.org> Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com> Cc: <linux-input@vger.kernel.org> Cc: Bryan Wu <bryan.wu@canonical.com> Cc: Richard Purdie <rpurdie@rpsys.net> Cc: <linux-leds@vger.kernel.org> Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de> Cc: <linux-fbdev@vger.kernel.org> Cc: Benoît Thébaudeau <benoit.thebaudeau@advansee.com> Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com> --- .../drivers/input/misc/pwm-beeper.c | 6 +++--- .../drivers/leds/leds-pwm.c | 2 +- .../drivers/video/backlight/pwm_bl.c | 2 +- 3 files changed, 5 insertions(+), 5 deletions(-)