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[86.9.19.6]) by smtp.gmail.com with ESMTPSA id k24sm7128343wmi.1.2019.10.09.03.42.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Oct 2019 03:42:37 -0700 (PDT) Date: Wed, 9 Oct 2019 11:42:36 +0100 From: Daniel Thompson To: Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= Cc: Enric Balletbo i Serra , linux-kernel@vger.kernel.org, thierry.reding@gmail.com, heiko@sntech.de, dianders@chromium.org, mka@chromium.org, groeck@chromium.org, kernel@collabora.com, bleung@chromium.org, linux-pwm@vger.kernel.org, Lee Jones Subject: Re: [PATCH] pwm: cros-ec: Let cros_ec_pwm_get_state() return the last applied state Message-ID: <20191009104236.ux23ywnhvsym2qcb@holly.lan> References: <20191008105417.16132-1-enric.balletbo@collabora.com> <20191008143432.pbhcqamd6f4qwbqn@pengutronix.de> <4f009344-242e-19a7-6872-2c55df086044@collabora.com> <20191008203137.s22clq6v2om5ktio@pengutronix.de> <53b7d02b-1a2d-11da-fdd0-5378f360d876@collabora.com> <20191009095635.yysr33lnwldicyng@holly.lan> <20191009101637.gmvghwdvcmfw4yyk@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20191009101637.gmvghwdvcmfw4yyk@pengutronix.de> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 09, 2019 at 12:16:37PM +0200, Uwe Kleine-König wrote: > On Wed, Oct 09, 2019 at 10:56:35AM +0100, Daniel Thompson wrote: > > On Wed, Oct 09, 2019 at 11:27:13AM +0200, Enric Balletbo i Serra wrote: > > > Hi Uwe, > > > > > > Adding Daniel and Lee to the discussion ... > > > > Thanks! > > > > > On 8/10/19 22:31, Uwe Kleine-König wrote: > > > > On Tue, Oct 08, 2019 at 06:33:15PM +0200, Enric Balletbo i Serra wrote: > > > >>> A few thoughts to your approach here ...: > > > >>> > > > >>> - Would it make sense to only store duty_cycle and enabled in the > > > >>> driver struct? > > > >>> > > > >> > > > >> Yes, in fact, my first approach (that I didn't send) was only storing enabled > > > >> and duty cycle. For some reason I ended storing the full pwm_state struct, but I > > > >> guess is not really needed. > > > >> > > > >> > > > >>> - Which driver is the consumer of your pwm? If I understand correctly > > > >>> the following sequence is the bad one: > > > >>> > > > >> > > > >> The consumer is the pwm_bl driver. Actually I'n trying to identify > > > >> other consumers. > > > > > > > > > > So far, the pwm_bl driver is the only consumer of cros-ec-pwm. > > > > > > > Ah, I see why I missed to identify the problem back when I checked this > > > > driver. The problem is not that .duty_cycle isn't set but there .enabled > > > > isn't set. So maybe we just want: > > > > > > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c > > > > index 2201b8c78641..0468c6ee4448 100644 > > > > --- a/drivers/video/backlight/pwm_bl.c > > > > +++ b/drivers/video/backlight/pwm_bl.c > > > > @@ -123,6 +123,7 @@ static int pwm_backlight_update_status(struct backlight_device *bl) > > > > if (brightness > 0) { > > > > pwm_get_state(pb->pwm, &state); > > > > state.duty_cycle = compute_duty_cycle(pb, brightness); > > > > + state.enabled = true; > > > > pwm_apply_state(pb->pwm, &state); > > > > pwm_backlight_power_on(pb); > > > > } else > > > > > > > > ? On a side note: It's IMHO strange that pwm_backlight_power_on > > > > reconfigures the PWM once more. > > > > > > > > > > Looking again to the pwm_bl code, now, I am not sure this is correct (although > > > it probably solves the problem for me). > > > > Looking at the pwm_bl code I wouldn't accept the above as it is but I'd > > almost certainly accept a patch to pwm_bl to move the PWM enable/disable > > out of both the power on/off functions so the duty-cycle/enable or > > disable can happen in one go within the update_status function. I don't > > think such a change would interfere with the power and enable sequencing > > needed by panels and it would therefore be a nice continuation of the > > work to convert over to the pwm_apply_state() API. > > OK for me. Enric, do you care enough to come up with a patch for pwm_bl? > I'd expect that this alone should already fix your issue. > > > None of the above has anything to do with what is right or wrong for > > the PWM API evolution. Of course, if this thread does conclude that it > > is OK the duty cycle of a disabled PWM to be retained for some drivers > > and not others then I'd hope to see some WARN_ON()s added to the PWM > > framework to help bring problems to the surface with all drivers. > > I think it's not possible to add a reliable WARN_ON for that issue. It > is quite expected that .get_state returns something that doesn't > completely match the requested configuration. So if a consumer requests > > .duty_cycle = 1 > .period = 100000000 > .enabled = false > > pwm_get_state possibly returns .duty_cycle = 0 even for drivers/hardware > that has a concept of duty_cycle for disabled hardware. > > A bit this is addressed in https://patchwork.ozlabs.org/patch/1147517/. Isn't that intended to help identify "odd" PWM drivers rather than "odd" clients? Initially I was thinking that a WARN_ON() could be emitted when: 1. .duty_cycle is non-zero 2. .enabled is false 3. the PWM is not already enabled (#3 included to avoid too many false positives when disabling a PWM) A poisoning approach might be equally valid. If some drivers are permitted to "round" .duty_cycle to 0 when .enabled is false then the framework could get *all* drivers to behave in the same way by zeroing it out before calling the drivers apply method. It is not that big a deal but minimising the difference between driver behaviour should automatically reduce the difference in API usage by clients. Daniel.