[Bug 203905] amdgpu:actual_brightness has unreal/wrong value

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=203905

--- Comment #3 from Lukáš Krejčí (lskrejci at gmail.com) ---
Commit 262485a50fd4 ("drm/amd/display: Expand dc to use 16.16 bit backlight")
changed the value that is returned for `actual_brightness`:

-static unsigned int dce_abm_get_current_backlight_8_bit(struct abm *abm)
+static unsigned int dce_abm_get_current_backlight(struct abm *abm)
 {
    struct dce_abm *abm_dce = TO_DCE_ABM(abm);
    unsigned int backlight = REG_READ(BL1_PWM_CURRENT_ABM_LEVEL);

-   return (backlight >> 8);
+   /* return backlight in hardware format which is unsigned 17 bits, with
+    * 1 bit integer and 16 bit fractional
+    */
+   return backlight;
 }


I could not find anything official about the range of `actual_brightness`, only
the following blog says that is has to be in range of `brightness`:
https://nikula.org/~jani/backlight/#index3h3

I guess amdgpu_dm_backlight_get_brightness() could be changed to bring the
`actual_brightness` value in range of `brightness` again since it returns the
value of `brightness` in the error path anyway.

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