Re: [PATCH i-g-t] tests/amdgpu: Discover amdgpu backlight device dynamically instead of hardcoding amdgpu_bl0

[Tom Chung <chiahsuan.chung@amd.com>]

Patch looks good to me.

Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>

On 6/26/2026 11:21 AM, James Lin wrote:
> [Why]
> amd_abm builds the backlight sysfs path assuming the amdgpu backlight
> device is amdgpu_bl0. The amdgpu backlight device is named amdgpu_bl<N>
> where N is derived from the DRM primary node index (see
> amdgpu_dm_backlight.c). On systems where the amdgpu DRM node is not
> card0 (e.g. simpledrm takes card0 and amdgpu comes up as card1), the
> backlight device is amdgpu_bl1, not amdgpu_bl0. The hardcoded
> "/sys/class/backlight/amdgpu_bl0" path does not exist, so
> backlight_read_max_brightness() and backlight_write_brightness() fail
> before any test logic runs, preventing dpms_cycle and the other abm
> subtests from executing.
>
> [How]
> Add find_backlight_device() to scan /sys/class/backlight for the first
> entry beginning with "amdgpu_bl" and store its full path in a
> file-scope buffer. Call it from igt_fixture and route the brightness
> and max_brightness accessors through the discovered path instead of the
> hardcoded amdgpu_bl0 macro. Use igt_require_f() so a machine with no
> amdgpu backlight skips cleanly rather than asserting.
>
> Signed-off-by: James Lin <PingLei.Lin@amd.com>
> ---
>   tests/amdgpu/amd_abm.c | 39 ++++++++++++++++++++++++++++++++++++---
>   1 file changed, 36 insertions(+), 3 deletions(-)
>
> diff --git a/tests/amdgpu/amd_abm.c b/tests/amdgpu/amd_abm.c
> index ba90344d3..7992dfd87 100644
> --- a/tests/amdgpu/amd_abm.c
> +++ b/tests/amdgpu/amd_abm.c
> @@ -31,13 +31,44 @@
>   #include <string.h>
>   #include <fcntl.h>
>   #include <time.h>
> +#include <dirent.h>
>   
>   #define DEBUGFS_CURRENT_BACKLIGHT_PWM "amdgpu_current_backlight_pwm"
>   #define DEBUGFS_TARGET_BACKLIGHT_PWM "amdgpu_target_backlight_pwm"
> -#define BACKLIGHT_PATH "/sys/class/backlight/amdgpu_bl0"
> +#define BACKLIGHT_DIR "/sys/class/backlight"
>   #define PANEL_POWER_SAVINGS_ATTR "amdgpu/panel_power_savings"
>   #define MK_COLOR(r, g, b)	((0 << 24) | (r << 16) | (g << 8) | b)
>   
> +/*
> + * The amdgpu backlight device is named amdgpu_bl<N> where N is derived from
> + * the DRM primary node index (see amdgpu_dm_backlight.c). On systems where
> + * the amdgpu DRM node is not card0 (e.g. card1), the device is amdgpu_bl1,
> + * not amdgpu_bl0. Discover it at runtime instead of hardcoding the name.
> + */
> +static char backlight_path[PATH_MAX];
> +
> +static void find_backlight_device(void)
> +{
> +	DIR *dir;
> +	struct dirent *ent;
> +
> +	dir = opendir(BACKLIGHT_DIR);
> +	igt_require_f(dir, "Cannot open %s\n", BACKLIGHT_DIR);
> +
> +	while ((ent = readdir(dir))) {
> +		if (strncmp(ent->d_name, "amdgpu_bl", 9) == 0) {
> +			snprintf(backlight_path, sizeof(backlight_path),
> +				 "%s/%s", BACKLIGHT_DIR, ent->d_name);
> +			break;
> +		}
> +	}
> +	closedir(dir);
> +
> +	igt_require_f(backlight_path[0] != '\0',
> +		      "No amdgpu_bl* backlight device found in %s\n",
> +		      BACKLIGHT_DIR);
> +}
> +
>   typedef struct data {
>   	igt_display_t display;
>   	igt_plane_t *primary;
> @@ -234,7 +265,7 @@ static int backlight_write_brightness(int value)
>   	char src[64];
>   	int len;
>   
> -	igt_assert(snprintf(full, PATH_MAX, "%s/%s", BACKLIGHT_PATH, "brightness") < PATH_MAX);
> +	igt_assert(snprintf(full, PATH_MAX, "%s/%s", backlight_path, "brightness") < PATH_MAX);
>   	fd = open(full, O_WRONLY);
>   	if (fd == -1)
>   		return -errno;
> @@ -288,7 +319,7 @@ static int backlight_read_max_brightness(int *result)
>   	char dst[64];
>   	int r, e;
>   
> -	igt_assert(snprintf(full, PATH_MAX, "%s/%s", BACKLIGHT_PATH, "max_brightness") < PATH_MAX);
> +	igt_assert(snprintf(full, PATH_MAX, "%s/%s", backlight_path, "max_brightness") < PATH_MAX);
>   
>   	fd = open(full, O_RDONLY);
>   	if (fd == -1)
> @@ -528,6 +559,8 @@ int igt_main()
>   
>   		igt_display_require(&data.display, data.drm_fd);
>   
> +		find_backlight_device();
> +
>   		test_init(&data);
>   	}
>