linux-kernel - Re: [PATCH v8 0/2] Add driver for Sharp Memory LCD

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Message-ID: <seex6vou5hig2qguejt7hfi5s5muikxch64f2d5fh2kcbs4tau@332xy75kvobl>
Date: Thu, 3 Oct 2024 11:45:51 -0400
From: Alex Lanzano <lanzano.alex@...il.com>
To: Uwe Kleine-König <u.kleine-koenig@...libre.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, Mehdi Djait <mehdi.djait@...tlin.com>, skhan@...uxfoundation.org, 
	linux-kernel-mentees@...ts.linuxfoundation.org, dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v8 0/2] Add driver for Sharp Memory LCD

On Thu, Oct 03, 2024 at 11:27:43AM GMT, Uwe Kleine-König wrote:
> Hello Alex,
> 
> On Wed, Oct 02, 2024 at 10:33:13PM -0400, Alex Lanzano wrote:
> > On Wed, Oct 02, 2024 at 09:56:38AM GMT, Uwe Kleine-König wrote:
> > > On Tue, Oct 01, 2024 at 11:37:35PM -0400, Alex Lanzano wrote:
> > > > Changes in v8:
> > > > - Addressed review comments from Uwe
> > > >     - Replace pwm_get_state with pwm_init_state
> > > >     - Use pwm_set_relative_duty_cycle instead of manually setting period and duty cycle
> > > 
> > > You didn't explicitly mention that it's fine if the PWM doesn't emit the
> > > inactive state when you call pwm_disable(). You're code should continue
> > > to work if you drop all calls to pwm_disable().
> > > 
> > > Ideally you mention that in a code comment to make others reading your
> > > code understand that.
> > 
> > Sorry about that! The intent of the code is to stop the pwm from outputing
> > when the display is disabled since the signal is no longer needed. If
> > it's best to emit the inactive state rather than calling pwm_disable()
> > I'm fine with making that change.
> 
> Calling pwm_disable() is best iff you don't care about the output any
> more. If however you rely on it to emit the inactive level,
> pwm_disable() is wrong. I don't know enough about your display to judge
> from here.
> 
> The code to disable the display looks (simplified) as follows:
> 
> 	if (smd->enable_gpio)
> 		gpiod_set_value(smd->enable_gpio, 0);
> 
> 	pwm_disable(smd->pwm_vcom_signal);
> 
> so maybe the logic you need is:
> 
> 	if (smd->enable_gpio) {
> 		gpiod_set_value(smd->enable_gpio, 0);
> 
> 		/*
> 		 * In the presence of a GPIO to disable the display the
> 		 * behaviour of the PWM doesn't matter and we can
> 		 * just disable it.
> 		 */
> 		pwm_disable(smd->pwm_vcom_signal);
> 	} else {
> 		struct pwm_state state;
> 
> 		/*
> 		 * However without a GPIO driving the display's output
> 		 * enable pin the PWM must emit the inactive level,
> 		 * which isn't guaranteed when calling pwm_disable(), so
> 		 * configure it for duty_cycle = 0.
> 		 */
> 		 pwm_init_state(smd->pwm_vcom_signal, &state);
> 		 state.duty_cycle = 0;
> 		 state.enabled = true;
> 		 pwm_apply_might_sleep(smd->pwm_vcom_signal, &state);
> 	}
> 

Ahh, understood. I looked over the datasheet again. It looks like in the
case where no enable gpio pin is defined we'll need to keep the pwm pulsing
since display hardware would still be enabled.

I'll fix this up in the disable and probe functions.

Best regards,
Alex