[Intel-gfx] [RFC PATCH] drm/i915: avoid round-trip scaling errors in actual_brightness
Eoff, Ullysses A
ullysses.a.eoff at intel.com
Thu Nov 20 18:33:52 CET 2014
> -----Original Message-----
> From: Nikula, Jani
> Sent: Thursday, November 20, 2014 3:59 AM
> To: intel-gfx at lists.freedesktop.org
> Cc: Nikula, Jani; Eoff, Ullysses A
> Subject: [RFC PATCH] drm/i915: avoid round-trip scaling errors in actual_brightness
>
> Due to scaling, the userspace and hardware brightness ranges might not
> have a 1:1 mapping, causing the backlight class sysfs actual_brightness
> not match the brightness attribute just written. While this is not a
> strict requirement per Documentation/ABI/stable/sysfs-class-backlight,
> try the userspace->hardware scaling for a match first and return the
> cached value to not confuse userspace.
>
> The problem was already mitigated by
>
> commit 673e7bbdb3920b62cfc6c710bea626b0a9b0f43a
> Author: U. Artie Eoff <ullysses.a.eoff at intel.com>
> Date: Mon Sep 29 15:49:32 2014 -0700
>
> drm/i915: intel_backlight scale() math WA
>
> but this should be more robust for cases where the userspace expects
> actual_brightness to match the just written brightness.
>
> Reference: http://mid.gmane.org/1415737838-9640-1-git-send-email-ullysses.a.eoff@intel.com
> Reference: https://bugzilla.kernel.org/show_bug.cgi?id=85861
> Cc: U. Artie Eoff <ullysses.a.eoff at intel.com>
> Signed-off-by: Jani Nikula <jani.nikula at intel.com>
Tested-by: U. Artie Eoff <ullysses.a.eoff at intel.com>
> ---
> drivers/gpu/drm/i915/intel_panel.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index 4d63839bd9b4..675b56e105e7 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -1024,7 +1024,17 @@ static int intel_backlight_device_get_brightness(struct backlight_device *bd)
> drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
>
> hw_level = intel_panel_get_backlight(connector);
> - ret = scale_hw_to_user(connector, hw_level, bd->props.max_brightness);
> +
> + /*
> + * Check the userspace->hardware scaling for a match first to avoid
> + * scaling errors in the userspace->hardware->userspace round-trip.
> + */
> + if (hw_level == scale_user_to_hw(connector, bd->props.brightness,
> + bd->props.max_brightness))
> + ret = bd->props.brightness;
> + else
> + ret = scale_hw_to_user(connector, hw_level,
> + bd->props.max_brightness);
>
> drm_modeset_unlock(&dev->mode_config.connection_mutex);
> intel_runtime_pm_put(dev_priv);
> --
> 2.1.3
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