linux-kernel - Re: [PATCH] drm/sun4i: rgb: Enable panel after controller

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Date:   Fri, 23 Sep 2016 16:16:26 +0300
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Jonathan Liu <net147@...il.com>
Cc:     David Airlie <airlied@...ux.ie>, Chen-Yu Tsai <wens@...e.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/sun4i: rgb: Enable panel after controller

On Thu, Sep 22, 2016 at 08:03:31AM +1000, Jonathan Liu wrote:
> Hi Maxime,
> 
> On Thursday, 22 September 2016, Maxime Ripard <maxime.ripard@...e-electrons.
> com> wrote:
> 
> > On Wed, Sep 21, 2016 at 11:03:04PM +1000, Jonathan Liu wrote:
> > > The panel should be enabled after the controller so that the panel
> > > prepare/enable delays are properly taken into account. Similarly, the
> > > panel should be disabled before the controller so that the panel
> > > unprepare/disable delays are properly taken into account.
> > >
> > > This is useful for avoiding visual glitches.
> >
> > This is not really taking any delays into account, especially since
> > drm_panel_enable and prepare are suppose to block until their
> > operation is complete.
> 
> 
> drm_panel_prepare turns on power to the LCD using enable-gpios property of
> the panel and then blocks for prepare delay. The prepare delay for panel
> can be set to how long it takes between the time the panel is powered to
> when it is ready to receive images. If backlight property is specified the
> backlight will be off while the panel is powered on.
> 
> drm_panel_enable blocks for enable delay and then turns on the backlight.
> The enable delay can be set to how long it takes for panel to start making
> the image visible after receiving the first valid frame. For example if the
> panel starts off as white and the TFT takes some time to initialize to
> black before it shows the image being received.
> 
> Refer to drivers/gpu/drm/panel-panel.simple.c for details.

From drm_panel.h:

"""
* drm_panel_enable - enable a panel
* @panel: DRM panel
*
* Calling this function will cause the panel display drivers to be turned on
* and the backlight to be enabled. Content will be visible on screen after
* this call completes.
"""

"""
* drm_panel_prepare - power on a panel
* @panel: DRM panel
*
* Calling this function will enable power and deassert any reset signals to
* the panel. After this has completed it is possible to communicate with any
* integrated circuitry via a command bus.
"""

Those comments clearly says that the caller should not have to deal
with the delays, even more so by just moving calls around and hoping
that the code running in between is adding enough delay for the panel
to behave properly.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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