Hack for setting gamma curves using linux KMS API

Makefile

LIBDRM_CFLAGS := $(shell pkg-config --cflags libdrm)
LIBDRM_LIBS := $(shell pkg-config --libs libdrm)

.PHONY = all clean

all: setgamma

clean:
	rm setgamma *.o

setgamma.o: setgamma.c
	gcc -c -o setgamma.o ${LIBDRM_CFLAGS} setgamma.c

setgamma: setgamma.o
	gcc -o setgamma ${LIBDRM_LIBS} setgamma.o

setgamma.c

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#include <xf86drm.h>
#include <xf86drmMode.h>

#define MAXBRIGHT 65535

int main(int argc, char** argv) {
    int ret;
    unsigned int i, v, bright;
    int fd = open("/dev/dri/card0", O_RDWR | O_CLOEXEC);
    uint32_t saved_conn_id, encoder_id;
    drmModeRes *resources = NULL;
    drmModeConnector *connector = NULL;
    drmModeEncoder *encoder = NULL;
    drmModeCrtc *crtc = NULL;

    if (argc != 2) {
        printf("Usage: %s <brightness 0-%d>\n", argv[0], MAXBRIGHT);
        ret = 1;
        goto cleanup;
    }

    if (sscanf(argv[1], "%u", &bright) != 1) {
        printf("Error - argument not an integer\n");
        ret = 1;
        goto cleanup;
    }
    
    if (bright > MAXBRIGHT) {
        printf("Error - brightness must be 0-%d\n", MAXBRIGHT);
        ret = 1;
        goto cleanup;
    }

    /* Find the first available connector with modes */
    resources = drmModeGetResources(fd);
    if (!resources) {
        printf("drmModeGetResources(%d) failed\n", fd);
        goto cleanup;
    }

    for (i = 0; i < resources->count_connectors; i++) {
        connector = drmModeGetConnector(fd, resources->connectors[i]);
        if (connector == NULL)
            continue;

        if (connector->connection == DRM_MODE_CONNECTED &&
            connector->count_modes > 0) {
            printf("Found connector %d with %d modes.\n",
                   connector->connector_id, connector->count_modes);
            saved_conn_id = connector->connector_id;
            break;
        }

        drmModeFreeConnector(connector);
    }

    if (i == resources->count_connectors) {
        printf("No currently active connector found.\n");
        goto cleanup;
    }

    for (i = 0; i < resources->count_encoders; i++) {
        encoder = drmModeGetEncoder(fd, resources->encoders[i]);

        if (encoder == NULL)
            continue;

        if (encoder->encoder_id == connector->encoder_id) {
            printf("Found encoder %d.\n", encoder->encoder_id);
            encoder_id = encoder->encoder_id;
            break;
        }

        drmModeFreeEncoder(encoder);
    }

    if (i == resources->count_encoders) {
        printf("No connected encoder found.\n");
        goto cleanup;
    }

    crtc = drmModeGetCrtc(fd, encoder->crtc_id);
    if (crtc == NULL) {
        printf("No CRTC found for encoder.\n");
        goto cleanup;
    }
    
    printf("Found connector ID %d, encoder ID %d, CRTC ID %d.\n", saved_conn_id, encoder_id, encoder->crtc_id);

    printf("Gamma size: %d\n", crtc->gamma_size);
    uint16_t *red = malloc(sizeof(uint16_t) * crtc->gamma_size);
    uint16_t *green = malloc(sizeof(uint16_t) * crtc->gamma_size);
    uint16_t *blue = malloc(sizeof(uint16_t) * crtc->gamma_size);
    ret = drmModeCrtcGetGamma(fd, encoder->crtc_id, crtc->gamma_size, red, green, blue);
    if(ret < 0) {
        printf("Error getting gamma: %d\n", ret);
        goto cleanup;
    }
    
    printf("Previous table:\n");
    for (i = 0; i < crtc->gamma_size; i++) {
        printf("%5d,%5d,%5d\n", red[i], green[i], blue[i]);
    }

    for (i = 0; i < crtc->gamma_size; i++) {
        v = (i * bright) / (crtc->gamma_size - 1);
        red[i] = green[i] = blue[i] = v;
    }
    ret = drmModeCrtcSetGamma(fd, encoder->crtc_id, crtc->gamma_size, red, green, blue);
    if (ret < 0) {
        printf("error setting gamma: %d\n", ret);
        goto cleanup;
    }

    printf("Current table\n");
    for (i = 0; i < crtc->gamma_size; i++) {
        printf("%5d,%5d,%5d\n", red[i], green[i], blue[i]);
    }

cleanup:
    close(fd);
    return ret;
}