[PATCH] drm/sun4i: rgb: Enable panel after controller

[Jonathan Liu]

Hi Maxime,

On 23 September 2016 at 23:16, Maxime Ripard
<maxime.ripard at free-electrons.com> wrote:
> On Thu, Sep 22, 2016 at 08:03:31AM +1000, Jonathan Liu wrote:
>> Hi Maxime,
>>
>> On Thursday, 22 September 2016, Maxime Ripard <maxime.ripard at free-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

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/drm/drm_panel.h#n34

In comment for struct drm_panel_funcs:
/**
 * struct drm_panel_funcs - perform operations on a given panel
 * @disable: disable panel (turn off back light, etc.)
 * @unprepare: turn off panel
 * @prepare: turn on panel and perform set up
 * @enable: enable panel (turn on back light, etc.)
 * @get_modes: add modes to the connector that the panel is attached to and
 * return the number of modes added
 * @get_timings: copy display timings into the provided array and return
 * the number of display timings available
 *
 * The .prepare() function is typically called before the display controller
 * starts to transmit video data. Panel drivers can use this to turn the panel
 * on and wait for it to become ready. If additional configuration is required
 * (via a control bus such as I2C, SPI or DSI for example) this is a good time
 * to do that.
 *
 * After the display controller has started transmitting video data, it's safe
 * to call the .enable() function. This will typically enable the backlight to
 * make the image on screen visible. Some panels require a certain amount of
 * time or frames before the image is displayed. This function is responsible
 * for taking this into account before enabling the backlight to avoid visual
 * glitches.
 *
 * Before stopping video transmission from the display controller it can be
 * necessary to turn off the panel to avoid visual glitches. This is done in
 * the .disable() function. Analogously to .enable() this typically involves
 * turning off the backlight and waiting for some time to make sure no image
 * is visible on the panel. It is then safe for the display controller to
 * cease transmission of video data.
 *
 * To save power when no video data is transmitted, a driver can power down
 * the panel. This is the job of the .unprepare() function.
 */

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/panel/panel-simple.c?id=refs/tags/v4.8-rc7#n39

In struct panel_desc:
/**
 * @prepare: the time (in milliseconds) that it takes for the panel to
 *           become ready and start receiving video data
 * @enable: the time (in milliseconds) that it takes for the panel to
 *          display the first valid frame after starting to receive
 *          video data
 * @disable: the time (in milliseconds) that it takes for the panel to
 *           turn the display off (no content is visible)
 * @unprepare: the time (in milliseconds) that it takes for the panel
 *             to power itself down completely
 */

Regards,
Jonathan