[PATCH 1/2] drm/i915: intel_backlight scale() math WA

[PATCH 1/2] drm/i915: intel_backlight scale() math WA

[U. Artie Eoff]

Improper truncated integer division in the scale() function causes
actual_brightness != brightness. This (partial) work-around should be
sufficient for a majority of use-cases, but it is by no means a complete
solution.

TODO: Determine how best to scale "user" values to "hw" values, and
vice-versa, when the ranges are of different sizes. That would be a
buggy scenario even with this work-around.

The issue was introduced in the following (v3.17-rc1) commit:

    6dda730 drm/i915: respect the VBT minimum backlight brightness

OTC-Jira: VIZ-4395

v2: (thanks to Chris Wilson) clarify commit message, use rounded division
macro

v3: -DIV_ROUND_CLOSEST() fails to build with CONFIG_X86_32=y. (Jani)
    -Use DIV_ROUND_CLOSEST_ULL() instead. (Damien)
    -v1 and v2 originally authored by Joe Konno.

v4: Add reference to issue report in commit message.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
---
 drivers/gpu/drm/i915/intel_panel.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index f17ada3..f7da913 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -398,6 +398,9 @@ intel_panel_detect(struct drm_device *dev)
 	}
 }
 
+#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
+({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
+
 /**
  * scale - scale values from one range to another
  *
@@ -419,9 +422,8 @@ static uint32_t scale(uint32_t source_val,
 	source_val = clamp(source_val, source_min, source_max);
 
 	/* avoid overflows */
-	target_val = (uint64_t)(source_val - source_min) *
-		(target_max - target_min);
-	do_div(target_val, source_max - source_min);
+	target_val = DIV_ROUND_CLOSEST_ULL((uint64_t)(source_val - source_min) *
+			(target_max - target_min), source_max - source_min);
 	target_val += target_min;
 
 	return target_val;
-- 
1.9.3

[PATCH 2/2 v2] drm/i915: Move DIV_ROUND_CLOSEST_ULL macro to header

[U. Artie Eoff]

Move the duplicated DIV_ROUND_CLOSEST_ULL macro into the intel_drv.h
header file so that it can be shared between intel_display.c
and intel_panel.c.

OTC-Jira: VIZ-4395

v2: Add reference to issue report in commit message.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 3 ---
 drivers/gpu/drm/i915/intel_drv.h     | 3 +++
 drivers/gpu/drm/i915/intel_panel.c   | 3 ---
 3 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 5b05ddb..db800f2 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -73,9 +73,6 @@ static const uint32_t intel_cursor_formats[] = {
 	DRM_FORMAT_ARGB8888,
 };
 
-#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
-({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
-
 static void intel_increase_pllclock(struct drm_device *dev,
 				    enum pipe pipe);
 static void intel_crtc_update_cursor(struct drm_crtc *crtc, bool on);
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 1b72c15..8401ae3 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -36,6 +36,9 @@
 #include <drm/drm_dp_mst_helper.h>
 #include <drm/drm_rect.h>
 
+#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
+({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
+
 /**
  * _wait_for - magic (register) wait macro
  *
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index f7da913..fade0dd 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -398,9 +398,6 @@ intel_panel_detect(struct drm_device *dev)
 	}
 }
 
-#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
-({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
-
 /**
  * scale - scale values from one range to another
  *
-- 
1.9.3

Re: [PATCH 1/2] drm/i915: intel_backlight scale() math WA

[Eoff, Ullysses A]

Ah snap... I forgot the v4 in the subject line :-/

Nonetheless, this is it.

On Tue, 2014-09-30 at 08:14 -0700, U. Artie Eoff wrote:
> Improper truncated integer division in the scale() function causes
> actual_brightness != brightness. This (partial) work-around should be
> sufficient for a majority of use-cases, but it is by no means a complete
> solution.
> 
> TODO: Determine how best to scale "user" values to "hw" values, and
> vice-versa, when the ranges are of different sizes. That would be a
> buggy scenario even with this work-around.
> 
> The issue was introduced in the following (v3.17-rc1) commit:
> 
>     6dda730 drm/i915: respect the VBT minimum backlight brightness
> 
> OTC-Jira: VIZ-4395
> 
> v2: (thanks to Chris Wilson) clarify commit message, use rounded division
> macro
> 
> v3: -DIV_ROUND_CLOSEST() fails to build with CONFIG_X86_32=y. (Jani)
>     -Use DIV_ROUND_CLOSEST_ULL() instead. (Damien)
>     -v1 and v2 originally authored by Joe Konno.
> 
> v4: Add reference to issue report in commit message.
> 
> Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f17ada3..f7da913 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -398,6 +398,9 @@ intel_panel_detect(struct drm_device *dev)
>  	}
>  }
>  
> +#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
> +({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
> +
>  /**
>   * scale - scale values from one range to another
>   *
> @@ -419,9 +422,8 @@ static uint32_t scale(uint32_t source_val,
>  	source_val = clamp(source_val, source_min, source_max);
>  
>  	/* avoid overflows */
> -	target_val = (uint64_t)(source_val - source_min) *
> -		(target_max - target_min);
> -	do_div(target_val, source_max - source_min);
> +	target_val = DIV_ROUND_CLOSEST_ULL((uint64_t)(source_val - source_min) *
> +			(target_max - target_min), source_max - source_min);
>  	target_val += target_min;
>  
>  	return target_val;