Re: [PATCH v5 18/20] drm/panel: panel-simple: Cache the EDID as long as we retain power

[Sean Paul <seanpaul@chromium.org>]

On Fri, Apr 23, 2021 at 1:00 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> It doesn't make sense to go out to the bus and read the EDID over and
> over again. Let's cache it and throw away the cache when we turn power
> off from the panel. Autosuspend means that even if there are several
> calls to read the EDID before we officially turn the power on then we
> should get good use out of this cache.
>

I think i915 caches the edid once on init and never refreshes it
(assuming no hotplugs). That said, I think it makes sense for a more
conservative approach in panel-simple.


> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>
> (no changes since v1)
>
>  drivers/gpu/drm/panel/panel-simple.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index a12dfe8b8d90..9be050ab372f 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -189,6 +189,8 @@ struct panel_simple {
>         struct gpio_desc *enable_gpio;
>         struct gpio_desc *hpd_gpio;
>
> +       struct edid *edid;
> +
>         struct drm_display_mode override_mode;
>
>         enum drm_panel_orientation orientation;
> @@ -345,6 +347,9 @@ static int panel_simple_suspend(struct device *dev)
>         regulator_disable(p->supply);
>         p->unprepared_time = ktime_get();
>
> +       kfree(p->edid);
> +       p->edid = NULL;
> +
>         return 0;
>  }
>
> @@ -510,15 +515,13 @@ static int panel_simple_get_modes(struct drm_panel *panel,
>
>         /* probe EDID if a DDC bus is available */
>         if (p->ddc) {
> -               struct edid *edid;
> -
>                 pm_runtime_get_sync(panel->dev);
>
> -               edid = drm_get_edid(connector, p->ddc);
> -               if (edid) {
> -                       num += drm_add_edid_modes(connector, edid);
> -                       kfree(edid);
> -               }
> +               if (!p->edid)
> +                       p->edid = drm_get_edid(connector, p->ddc);
> +
> +               if (p->edid)
> +                       num += drm_add_edid_modes(connector, p->edid);

I suppose this would keep banging on the ddc if drm_get_edid()
continuously returns NULL, but maybe that's desireable (it'll succeed
sometime in the future)? At any rate, this is an improvement on
current behavior so it has my vote.

Reviewed-by: Sean Paul <seanpaul@chromium.org>

>
>                 pm_runtime_mark_last_busy(panel->dev);
>                 pm_runtime_put_autosuspend(panel->dev);
> --
> 2.31.1.498.g6c1eba8ee3d-goog
>
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