Re: [PATCH v3 12/12] drm/panel: panel-simple: Use runtime pm to avoid excessive unprepare / prepare

Laurent Pinchart
2021-04-15T00:58:50Z

Hi Doug,

Thank you for the patch.

On Fri, Apr 02, 2021 at 03:28:46PM -0700, Douglas Anderson wrote:
> Unpreparing and re-preparing a panel can be a really heavy
> operation. Panels datasheets often specify something on the order of
> 500ms as the delay you should insert after turning off the panel
> before turning it on again. In addition, turning on a panel can have
> delays on the order of 100ms - 200ms before the panel will assert HPD
> (AKA "panel ready"). The above means that we should avoid turning a
> panel off if we're going to turn it on again shortly.
>
> The above becomes a problem when we want to read the EDID of a
> panel. The way that ordering works is that userspace wants to read the
> EDID of the panel _before_ fully enabling it so that it can set the
> initial mode correctly. However, we can't read the EDID until we power
> it up. This leads to code that does this dance (like
> ps8640_bridge_get_edid()):
>
> 1. When userspace requests EDID / the panel modes (through an ioctl),
> we power on the panel just enough to read the EDID and then power
> it off.
> 2. Userspace then turns the panel on.
>
> There's likely not much time between step #1 and #2 and so we want to
> avoid powering the panel off and on again between those two steps.
>
> Let's use Runtime PM to help us. We'll move the existing prepare() and
> unprepare() to be runtime resume() and runtime suspend(). Now when we
> want to prepare() or unprepare() we just increment or decrement the
> refcount. We'll default to a 1 second autosuspend delay which seems
> sane given the typical delays we see for panels.
>
> A few notes:
> - It seems the existing unprepare() and prepare() are defined to be
> no-ops if called extra times. We'll preserve that behavior.

The prepare and unprepare calls are supposed to be balanced, which
should allow us to drop this check. Do you have a reason to suspect that
it may not be the case ?

> - This is a slight change in the ABI of simple panel. If something was
> absolutely relying on the unprepare() to happen instantly that
> simply won't be the case anymore. I'm not aware of anyone relying on
> that behavior, but if there is someone then we'll need to figure out
> how to enable (or disable) this new delayed behavior selectively.
> - In order for this to work we now have a hard dependency on
> "PM". From memory this is a legit thing to assume these days and we
> don't have to find some fallback to keep working if someone wants to
> build their system without "PM".

Sounds fine to me.

The code looks good to me. Possibly with the prepared check removed,

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> (no changes since v1)
>
> drivers/gpu/drm/panel/Kconfig | 1 +
> drivers/gpu/drm/panel/panel-simple.c | 93 +++++++++++++++++++++-------
> 2 files changed, 73 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/gpu/drm/panel/Kconfig b/drivers/gpu/drm/panel/Kconfig
> index 4894913936e9..ef87d92cdf49 100644
> --- a/drivers/gpu/drm/panel/Kconfig
> +++ b/drivers/gpu/drm/panel/Kconfig
> @@ -80,6 +80,7 @@ config DRM_PANEL_SIMPLE
> tristate "support for simple panels"
> depends on OF
> depends on BACKLIGHT_CLASS_DEVICE
> + depends on PM
> select VIDEOMODE_HELPERS
> help
> DRM panel driver for dumb panels that need at most a regulator and
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index be312b5c04dd..6b22872b3281 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -27,6 +27,7 @@
> #include <linux/module.h>
> #include <linux/of_platform.h>
> #include <linux/platform_device.h>
> +#include <linux/pm_runtime.h>
> #include <linux/regulator/consumer.h>
>
> #include <video/display_timing.h>
> @@ -175,6 +176,8 @@ struct panel_simple {
> bool enabled;
> bool no_hpd;
>
> + bool prepared;
> +
> ktime_t prepared_time;
> ktime_t unprepared_time;
>
> @@ -334,19 +337,31 @@ static int panel_simple_disable(struct drm_panel *panel)
> return 0;
> }
>
> +static int panel_simple_suspend(struct device *dev)
> +{
> + struct panel_simple *p = dev_get_drvdata(dev);
> +
> + gpiod_set_value_cansleep(p->enable_gpio, 0);
> + regulator_disable(p->supply);
> + p->unprepared_time = ktime_get();
> +
> + return 0;
> +}
> +
> static int panel_simple_unprepare(struct drm_panel *panel)
> {
> struct panel_simple *p = to_panel_simple(panel);
> + int ret;
>
> - if (p->prepared_time == 0)
> + /* Unpreparing when already unprepared is a no-op */
> + if (!p->prepared)
> return 0;
>
> - gpiod_set_value_cansleep(p->enable_gpio, 0);
> -
> - regulator_disable(p->supply);
> -
> - p->prepared_time = 0;
> - p->unprepared_time = ktime_get();
> + pm_runtime_mark_last_busy(panel->dev);
> + ret = pm_runtime_put_autosuspend(panel->dev);
> + if (ret < 0)
> + return ret;
> + p->prepared = false;
>
> return 0;
> }
> @@ -376,22 +391,19 @@ static int panel_simple_get_hpd_gpio(struct device *dev,
> return 0;
> }
>
> -static int panel_simple_prepare_once(struct drm_panel *panel)
> +static int panel_simple_prepare_once(struct panel_simple *p)
> {
> - struct panel_simple *p = to_panel_simple(panel);
> + struct device *dev = p->base.dev;
> unsigned int delay;
> int err;
> int hpd_asserted;
> unsigned long hpd_wait_us;
>
> - if (p->prepared_time != 0)
> - return 0;
> -
> panel_simple_wait(p->unprepared_time, p->desc->delay.unprepare);
>
> err = regulator_enable(p->supply);
> if (err < 0) {
> - dev_err(panel->dev, "failed to enable supply: %d\n", err);
> + dev_err(dev, "failed to enable supply: %d\n", err);
> return err;
> }
>
> @@ -405,7 +417,7 @@ static int panel_simple_prepare_once(struct drm_panel *panel)
>
> if (p->hpd_gpio) {
> if (IS_ERR(p->hpd_gpio)) {
> - err = panel_simple_get_hpd_gpio(panel->dev, p, false);
> + err = panel_simple_get_hpd_gpio(dev, p, false);
> if (err)
> goto error;
> }
> @@ -423,7 +435,7 @@ static int panel_simple_prepare_once(struct drm_panel *panel)
>
> if (err) {
> if (err != -ETIMEDOUT)
> - dev_err(panel->dev,
> + dev_err(dev,
> "error waiting for hpd GPIO: %d\n", err);
> goto error;
> }
> @@ -447,25 +459,46 @@ static int panel_simple_prepare_once(struct drm_panel *panel)
> */
> #define MAX_PANEL_PREPARE_TRIES 5
>
> -static int panel_simple_prepare(struct drm_panel *panel)
> +static int panel_simple_resume(struct device *dev)
> {
> + struct panel_simple *p = dev_get_drvdata(dev);
> int ret;
> int try;
>
> for (try = 0; try < MAX_PANEL_PREPARE_TRIES; try++) {
> - ret = panel_simple_prepare_once(panel);
> + ret = panel_simple_prepare_once(p);
> if (ret != -ETIMEDOUT)
> break;
> }
>
> if (ret == -ETIMEDOUT)
> - dev_err(panel->dev, "Prepare timeout after %d tries\n", try);
> + dev_err(dev, "Prepare timeout after %d tries\n", try);
> else if (try)
> - dev_warn(panel->dev, "Prepare needed %d retries\n", try);
> + dev_warn(dev, "Prepare needed %d retries\n", try);
>
> return ret;
> }
>
> +static int panel_simple_prepare(struct drm_panel *panel)
> +{
> + struct panel_simple *p = to_panel_simple(panel);
> + int ret;
> +
> + /* Preparing when already prepared is a no-op */
> + if (p->prepared)
> + return 0;
> +
> + ret = pm_runtime_get_sync(panel->dev);
> + if (ret < 0) {
> + pm_runtime_put_autosuspend(panel->dev);
> + return ret;
> + }
> +
> + p->prepared = true;
> +
> + return 0;
> +}
> +
> static int panel_simple_enable(struct drm_panel *panel)
> {
> struct panel_simple *p = to_panel_simple(panel);
> @@ -748,6 +781,18 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
> break;
> }
>
> + dev_set_drvdata(dev, panel);
> +
> + /*
> + * We use runtime PM for prepare / unprepare since those power the panel
> + * on and off and those can be very slow operations. This is important
> + * to optimize powering the panel on briefly to read the EDID before
> + * fully enabling the panel.
> + */
> + pm_runtime_enable(dev);
> + pm_runtime_set_autosuspend_delay(dev, 1000);
> + pm_runtime_use_autosuspend(dev);
> +
> drm_panel_init(&panel->base, dev, &panel_simple_funcs, connector_type);
>
> err = drm_panel_of_backlight(&panel->base);
> @@ -756,8 +801,6 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
>
> drm_panel_add(&panel->base);
>
> - dev_set_drvdata(dev, panel);
> -
> return 0;
>
> free_ddc:
> @@ -4603,10 +4646,17 @@ static void panel_simple_platform_shutdown(struct platform_device *pdev)
> panel_simple_shutdown(&pdev->dev);
> }
>
> +static const struct dev_pm_ops panel_simple_pm_ops = {
> + SET_RUNTIME_PM_OPS(panel_simple_suspend, panel_simple_resume, NULL)
> + SET_SYSTEM_SLEEP_PM_OPS(pm_runtime_force_suspend,
> + pm_runtime_force_resume)
> +};
> +
> static struct platform_driver panel_simple_platform_driver = {
> .driver = {
> .name = "panel-simple",
> .of_match_table = platform_of_match,
> + .pm = &panel_simple_pm_ops,
> },
> .probe = panel_simple_platform_probe,
> .remove = panel_simple_platform_remove,
> @@ -4901,6 +4951,7 @@ static struct mipi_dsi_driver panel_simple_dsi_driver = {
> .driver = {
> .name = "panel-simple-dsi",
> .of_match_table = dsi_of_match,
> + .pm = &panel_simple_pm_ops,
> },
> .probe = panel_simple_dsi_probe,
> .remove = panel_simple_dsi_remove,

--
Regards,

Laurent Pinchart