[PATCH 1/5] drm: Add helpers for programming hardware gamma LUTs
Thomas Zimmermann
tzimmermann at suse.de
Mon May 12 13:55:46 UTC 2025
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 <tzimmermann at 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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