Re: [PATCH 1/5] drm: Add helpers for programming hardware gamma LUTs

Mon, 12 May 2025 06:56:00 -0700 

Hi

Am 12.05.25 um 15:31 schrieb Jocelyn Falempe:

On 09/05/2025 10:23, Thomas Zimmermann wrote:

Provide helpers that program hardware gamma LUTs. Tha gamma ramp is
either provided by the driver or generated by the helper.

The DRM driver exports the GAMMA_LUT property with a fixed number of
entries per color component, such as 256 on 8-bit-wide components. The
entries describe the gamma ramp of each individual component. The new
helper drm_crtc_load_gamma_888() loads such gamma ramp to hardware. The
hardware uses each displayed pixel's individial components as indeces
into the hardware gamma table.

For color modes with less than 8 bits per color component, the helpers
drm_crtc_load_gamma_565_from() and drm_crtc_load_gamma_555_from_888()
interpolate the provided gamma ramp to reduce it to the correct number
of entries; 5/6/5 for RGB565-like formats and 5/5/5 for RGB1555-like
formats.

If no gamma ramp has been provided, drivers can use the new helper
drm_crtc_fill_gamma_888() to load a default gamma ramp with 256 entries
per color component. For color modes with less bits, the new helpers
drm_crtc_fill_gamma_565() and drm_crtc_fill_gamma_555() are available.
The default gamma ramp uses a gamma factor of 1. Later patches can
change this. For PCs, a gamma factor of 2.2 is common.

For color modes with palette, drm_crtc_load_palette_8() load an 8-bit
palette into the hardware. If no palette has been specified,
drm_crtc_fill_palette_8() load a system-specific default palette. This
is currently only a grey-scale palette with increasing luminance, but
later patches can change this. For PCs, a VGA default palette could
be used.
Thanks for this work. I've one comment below regarding the drm_crtc_set_lut_func prototype. 

Signed-off-by: Thomas Zimmermann <tzimmerm...@suse.de>
---
  drivers/gpu/drm/drm_color_mgmt.c | 206 +++++++++++++++++++++++++++++++
  include/drm/drm_color_mgmt.h     |  27 ++++
  2 files changed, 233 insertions(+)
diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c 
index 3969dc548cff..dd3e06605180 100644
--- a/drivers/gpu/drm/drm_color_mgmt.c
+++ b/drivers/gpu/drm/drm_color_mgmt.c
@@ -630,3 +630,209 @@ int drm_color_lut_check(const struct drm_property_blob *lut, u32 tests) 
      return 0;
  }
  EXPORT_SYMBOL(drm_color_lut_check);
+
+/*
+ * Gamma-LUT programming
+ */
+
+/**
+ * drm_crtc_load_gamma_888 - Programs gamma ramp for RGB888-like formats 
+ * @crtc: The displaying CRTC
+ * @lut: The gamma ramp to program
+ * @set_gamma: Callback for programming the hardware gamma LUT
+ *
+ * Programs the gamma ramp specified in @lut to hardware. The input gamma 
+ * ramp must have 256 entries per color component.
+ */
+void drm_crtc_load_gamma_888(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                 drm_crtc_set_lut_func set_gamma)
+{
+    unsigned int i;
+
+    for (i = 0; i < 256; ++i)
+        set_gamma(crtc, i, lut[i].red, lut[i].green, lut[i].blue);
+}
+EXPORT_SYMBOL(drm_crtc_load_gamma_888);
+
+/**
+ * drm_crtc_load_gamma_565_from_888 - Programs gamma ramp for RGB565-like formats 
+ * @crtc: The displaying CRTC
+ * @lut: The gamma ramp to program
+ * @set_gamma: Callback for programming the hardware gamma LUT
+ *
+ * Programs the gamma ramp specified in @lut to hardware. The input gamma + * ramp must have 256 entries per color component. The helper interpolates + * the individual color components to reduce the number of entries to 5/6/5. 
+ */
+void drm_crtc_load_gamma_565_from_888(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                      drm_crtc_set_lut_func set_gamma)
+{
+    unsigned int i;
+    u16 r, g, b;
+
+    for (i = 0; i < 32; ++i) {
+        r = lut[i * 8 + i / 4].red;
+        g = lut[i * 4 + i / 16].green;
+        b = lut[i * 8 + i / 4].blue;
+        set_gamma(crtc, i, r, g, b);
+    }
+    /* Green has one more bit, so add padding with 0 for red and blue. */ 
+    for (i = 32; i < 64; ++i) {
+        g = lut[i * 4 + i / 16].green;
+        set_gamma(crtc, i, 0, g, 0);
+    }
+}
+EXPORT_SYMBOL(drm_crtc_load_gamma_565_from_888);
+
+/**
+ * drm_crtc_load_gamma_555_from_888 - Programs gamma ramp for RGB555-like formats 
+ * @crtc: The displaying CRTC
+ * @lut: The gamma ramp to program
+ * @set_gamma: Callback for programming the hardware gamma LUT
+ *
+ * Programs the gamma ramp specified in @lut to hardware. The input gamma + * ramp must have 256 entries per color component. The helper interpolates + * the individual color components to reduce the number of entries to 5/5/5. 
+ */
+void drm_crtc_load_gamma_555_from_888(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                      drm_crtc_set_lut_func set_gamma)
+{
+    unsigned int i;
+    u16 r, g, b;
+
+    for (i = 0; i < 32; ++i) {
+        r = lut[i * 8 + i / 4].red;
+        g = lut[i * 8 + i / 4].green;
+        b = lut[i * 8 + i / 4].blue;
+        set_gamma(crtc, i, r, g, b);
+    }
+}
+EXPORT_SYMBOL(drm_crtc_load_gamma_555_from_888);
+
+static void fill_gamma_888(struct drm_crtc *crtc, unsigned int i, u16 r, u16 g, u16 b, 
+               drm_crtc_set_lut_func set_gamma)
+{
+    r = (r << 8) | r;
+    g = (g << 8) | g;
+    b = (b << 8) | b;
+
+    set_gamma(crtc, i, r, g, b);
+}
+
+/**
+ * drm_crtc_fill_gamma_888 - Programs a default gamma ramp for RGB888-like formats 
+ * @crtc: The displaying CRTC
+ * @set_gamma: Callback for programming the hardware gamma LUT
+ *
+ * Programs a default gamma ramp to hardware.
+ */
+void drm_crtc_fill_gamma_888(struct drm_crtc *crtc, drm_crtc_set_lut_func set_gamma) 
+{
+    unsigned int i;
+
+    for (i = 0; i < 256; ++i)
+        fill_gamma_888(crtc, i, i, i, i, set_gamma);
+}
+EXPORT_SYMBOL(drm_crtc_fill_gamma_888);
+
+static void fill_gamma_565(struct drm_crtc *crtc, unsigned int i, u16 r, u16 g, u16 b, 
+               drm_crtc_set_lut_func set_gamma)
+{
+    r = (r << 11) | (r << 6) | (r << 1) | (r >> 4);
+    g = (g << 10) | (g << 4) | (g >> 2);
+    b = (b << 11) | (b << 6) | (b << 1) | (b >> 4);
+
+    set_gamma(crtc, i, r, g, b);
+}
+
+/**
+ * drm_crtc_fill_gamma_565 - Programs a default gamma ramp for RGB565-like formats 
+ * @crtc: The displaying CRTC
+ * @set_gamma: Callback for programming the hardware gamma LUT
+ *
+ * Programs a default gamma ramp to hardware.
+ */
+void drm_crtc_fill_gamma_565(struct drm_crtc *crtc, drm_crtc_set_lut_func set_gamma) 
+{
+    unsigned int i;
+
+    for (i = 0; i < 32; ++i)
+        fill_gamma_565(crtc, i, i, i, i, set_gamma);
+    /* Green has one more bit, so add padding with 0 for red and blue. */ 
+    for (i = 32; i < 64; ++i)
+        fill_gamma_565(crtc, i, 0, i, 0, set_gamma);
+}
+EXPORT_SYMBOL(drm_crtc_fill_gamma_565);
+
+static void fill_gamma_555(struct drm_crtc *crtc, unsigned int i, u16 r, u16 g, u16 b, 
+               drm_crtc_set_lut_func set_gamma)
+{
+    r = (r << 11) | (r << 6) | (r << 1) | (r >> 4);
+    g = (g << 11) | (g << 6) | (g << 1) | (g >> 4);
+    b = (b << 11) | (b << 6) | (b << 1) | (r >> 4);
+
+    set_gamma(crtc, i, r, g, b);
+}
+
+/**
+ * drm_crtc_fill_gamma_555 - Programs a default gamma ramp for RGB555-like formats 
+ * @crtc: The displaying CRTC
+ * @set_gamma: Callback for programming the hardware gamma LUT
+ *
+ * Programs a default gamma ramp to hardware.
+ */
+void drm_crtc_fill_gamma_555(struct drm_crtc *crtc, drm_crtc_set_lut_func set_gamma) 
+{
+    unsigned int i;
+
+    for (i = 0; i < 32; ++i)
+        fill_gamma_555(crtc, i, i, i, i, set_gamma);
+}
+EXPORT_SYMBOL(drm_crtc_fill_gamma_555);
+
+/*
+ * Color-LUT programming
+ */
+
+/**
+ * drm_crtc_load_palette_8 - Programs palette for C8-like formats
+ * @crtc: The displaying CRTC
+ * @lut: The palette to program
+ * @set_palette: Callback for programming the hardware palette
+ *
+ * Programs the palette specified in @lut to hardware. The input palette 
+ * must have 256 entries per color component.
+ */
+void drm_crtc_load_palette_8(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                 drm_crtc_set_lut_func set_palette)
+{
+    unsigned int i;
+
+    for (i = 0; i < 256; ++i)
+        set_palette(crtc, i, lut[i].red, lut[i].green, lut[i].blue);
+}
+EXPORT_SYMBOL(drm_crtc_load_palette_8);
+
+static void fill_palette_8(struct drm_crtc *crtc, unsigned int i,
+               drm_crtc_set_lut_func set_palette)
+{
+    u16 Y = (i << 8) | i; // relative luminance
+
+    set_palette(crtc, i, Y, Y, Y);
+}
+
+/**
+ * drm_crtc_fill_palette_8 - Programs a default palette for C8-like formats 
+ * @crtc: The displaying CRTC
+ * @set_palette: Callback for programming the hardware gamma LUT
+ *
+ * Programs a default palette to hardware.
+ */
+void drm_crtc_fill_palette_8(struct drm_crtc *crtc, drm_crtc_set_lut_func set_palette) 
+{
+    unsigned int i;
+
+    for (i = 0; i < 256; ++i)
+        fill_palette_8(crtc, i, set_palette);
+}
+EXPORT_SYMBOL(drm_crtc_fill_palette_8);
diff --git a/include/drm/drm_color_mgmt.h b/include/drm/drm_color_mgmt.h
index ed81741036d7..6cb577f6dba6 100644
--- a/include/drm/drm_color_mgmt.h
+++ b/include/drm/drm_color_mgmt.h
@@ -118,4 +118,31 @@ enum drm_color_lut_tests {
  };
    int drm_color_lut_check(const struct drm_property_blob *lut, u32 tests); 
+
+/*
+ * Gamma-LUT programming
+ */
+
+typedef void (*drm_crtc_set_lut_func)(struct drm_crtc *, unsigned int, u16, u16, u16); 

All drivers using this code, have 256 LUT size, and use 8bits value.
So I would prefer to have it declared like this:
typedef void (*drm_crtc_set_lut_func)(struct drm_crtc *, u8, u8, u8, u8);
This will avoid to upcast from u8 to u16, and then from u16 to u8 in each driver.
I don't want to change that, because that second shift really depends on the hardware:
- some of the VESA color modes affect bit shifts [1] (although I was unable to setup one so far) 
- anything VGA-compatible would use 6 bits per component
- other drivers use 10 bits per component, for example amdgpu at [2].

To build reusable helpers, we need to take this into account.

Best regards
Thomas
[1] https://elixir.bootlin.com/linux/v6.14.6/source/drivers/video/fbdev/vesafb.c#L93 [2] https://elixir.bootlin.com/linux/v6.14.6/source/drivers/gpu/drm/amd/amdgpu/dce_v8_0.c#L2071 




+
+void drm_crtc_load_gamma_888(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                 drm_crtc_set_lut_func set_gamma);
+void drm_crtc_load_gamma_565_from_888(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                      drm_crtc_set_lut_func set_gamma);
+void drm_crtc_load_gamma_555_from_888(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                      drm_crtc_set_lut_func set_gamma);
+
+void drm_crtc_fill_gamma_888(struct drm_crtc *crtc, drm_crtc_set_lut_func set_gamma); +void drm_crtc_fill_gamma_565(struct drm_crtc *crtc, drm_crtc_set_lut_func set_gamma); +void drm_crtc_fill_gamma_555(struct drm_crtc *crtc, drm_crtc_set_lut_func set_gamma); 
+
+/*
+ * Color-LUT programming
+ */
+
+void drm_crtc_load_palette_8(struct drm_crtc *crtc, const struct drm_color_lut *lut, 
+                 drm_crtc_set_lut_func set_palette);
+
+void drm_crtc_fill_palette_8(struct drm_crtc *crtc, drm_crtc_set_lut_func set_palette); 
+
  #endif



Best regards,



--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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