Re: [PATCH i-g-t] lib/igt_kms: Identify writeback connectors in msm driver on android

[Jani Nikula <jani.nikula@intel.com>]

On Tue, 07 Apr 2026, Harshdeep Dhatt <harshdeepdhatt@gmail.com> wrote:
> The msm drm driver that runs on android platforms sets up its writeback
> connectors as DRM_MODE_CONNECTOR_VIRTUAL instead of DRM_MODE_CONNECTOR_WRITEBACK.
> This means when setting up the drm resources, these connectors get exposed
> to userspace even though userspace has not explicitly asked for writeback
> connectors. These connectors are by default in the connected state. When we
> try to configure one of the crtc's for this output (to get the primary plane),
> the msm driver bails because this connector (being a writeback connector)
> doesn't have an output framebuffer assigned to it. To avoid this failure, get the
> correct property id for setting up the output framebuffer id for writeback connectors
> as defined by the msm driver on android. Allocate and bind a dummy output fb for
> these connectors.

Is this msm android driver in upstream linux kernel?

If not, why should we add this in upstream IGT?

BR,
Jani.


>
> Signed-off-by: Harshdeep Dhatt <harshdeepdhatt@gmail.com>
> ---
>  lib/drmtest.c |  5 +++++
>  lib/drmtest.h |  1 +
>  lib/igt_kms.c | 28 ++++++++++++++++++++++++++++
>  3 files changed, 34 insertions(+)
>
> diff --git a/lib/drmtest.c b/lib/drmtest.c
> index 4a788ea7a..2e6895618 100644
> --- a/lib/drmtest.c
> +++ b/lib/drmtest.c
> @@ -143,6 +143,11 @@ bool is_msm_device(int fd)
>  	return __is_device(fd, "msm");
>  }
>  
> +bool is_msm_android_device(int fd)
> +{
> +	return __is_device(fd, "msm_drm");
> +}
> +
>  bool is_nouveau_device(int fd)
>  {
>  	/* Currently all nouveau-specific codepaths require libdrm */
> diff --git a/lib/drmtest.h b/lib/drmtest.h
> index 37874d729..c042a2b20 100644
> --- a/lib/drmtest.h
> +++ b/lib/drmtest.h
> @@ -147,6 +147,7 @@ bool is_amdgpu_device(int fd);
>  bool is_i915_device(int fd);
>  bool is_mtk_device(int fd);
>  bool is_msm_device(int fd);
> +bool is_msm_android_device(int fd);
>  bool is_nouveau_device(int fd);
>  bool is_vc4_device(int fd);
>  bool is_xe_device(int fd);
> diff --git a/lib/igt_kms.c b/lib/igt_kms.c
> index 04b1f3972..3948426bc 100644
> --- a/lib/igt_kms.c
> +++ b/lib/igt_kms.c
> @@ -950,6 +950,14 @@ igt_atomic_fill_connector_props(igt_display_t *display, igt_output_t *output,
>  	for (i = 0; i < props->count_props; i++) {
>  		drmModePropertyPtr prop =
>  			drmModeGetProperty(fd, props->props[i]);
> +		/*
> +		 * msm android driver installs writeback fb id as a custom property which can be
> +		 * identified by "FB_ID" string
> +		 */
> +		if (is_msm_android_device(fd) && (!strcmp("FB_ID", prop->name))) {
> +			output->props[IGT_CONNECTOR_WRITEBACK_FB_ID] = props->props[i];
> +			continue;
> +		}
>  
>  		for (j = 0; j < num_connector_props; j++) {
>  			if (strcmp(prop->name, conn_prop_names[j]) != 0)
> @@ -1003,6 +1011,7 @@ static igt_plane_t *igt_get_assigned_primary(igt_output_t *output,
>  	int drm_fd = output->display->drm_fd;
>  	drmModeModeInfo *mode;
>  	struct igt_fb fb;
> +	struct igt_fb out_fb = {0};
>  	igt_plane_t *plane = NULL;
>  	uint32_t crtc_id;
>  	int i;
> @@ -1016,6 +1025,23 @@ static igt_plane_t *igt_get_assigned_primary(igt_output_t *output,
>  
>  	crtc_id = crtc->crtc_id;
>  
> +	/*
> +	 * msm android driver exposes writeback connectors as DRM_MODE_CONNECTOR_VIRTUAL and not
> +	 * DRM_MODE_CONNECTOR_WRITEBACK. The driver will bail if the output fb is not specified for
> +	 * this connector. Hence, allocate a dummy output fb here and assign it to the connector.
> +	 */
> +	if (is_msm_android_device(drm_fd) && igt_output_has_prop(output, IGT_CONNECTOR_WRITEBACK_FB_ID)) {
> +		igt_require(igt_create_fb(drm_fd, mode->hdisplay, mode->vdisplay,
> +			DRM_FORMAT_XRGB8888,
> +			DRM_FORMAT_MOD_NONE,
> +			&out_fb) == 0);
> +		igt_output_set_prop_value(output, IGT_CONNECTOR_WRITEBACK_FB_ID, out_fb.fb_id);
> +		igt_assert(drmModeConnectorSetProperty(drm_fd, output->id,
> +			output->props[IGT_CONNECTOR_WRITEBACK_FB_ID],
> +			output->values[IGT_CONNECTOR_WRITEBACK_FB_ID]) == 0);
> +	}
> +
> +
>  	/*
>  	 * Do a legacy SETCRTC to start things off, so that we know that
>  	 * the kernel will pick the correct primary plane and attach it
> @@ -1039,6 +1065,8 @@ static igt_plane_t *igt_get_assigned_primary(igt_output_t *output,
>  
>  	/* Removing the FB will also shut down the display for us: */
>  	igt_remove_fb(drm_fd, &fb);
> +	if (out_fb.fb_id != 0)
> +		igt_remove_fb(drm_fd, &out_fb);
>  	igt_assert_f(plane, "Valid assigned primary plane for CRTC_ID %d not found.\n", crtc_id);
>  
>  	return plane;

-- 
Jani Nikula, Intel