Nouveau - Feb 2022 - [PATCH] drm/nouveau/backlight: Fix LVDS backlight detection on some laptops

It seems that some laptops will report having both an eDP and LVDS
connector, even though only the LVDS connector is actually hooked up. This
can lead to issues with backlight registration if the eDP connector ends up
getting registered before the LVDS connector, as the backlight device will
then be registered to the eDP connector instead of the LVDS connector.

So, fix this by only registering the backlight on connectors that are
reported as being connected.

Signed-off-by: Lyude Paul <lyude at redhat.com>
Fixes: 6eca310e8924 ("drm/nouveau/kms/nv50-: Add basic DPCD backlight
support for nouveau")
Bugzilla: https://gitlab.freedesktop.org/drm/nouveau/-/issues/137
Cc: <stable at vger.kernel.org> # v5.15+
---
 drivers/gpu/drm/nouveau/nouveau_backlight.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
b/drivers/gpu/drm/nouveau/nouveau_backlight.c
index ae2f2abc8f5a..6af12dc99d7f 100644
--- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
+++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
@@ -294,7 +294,8 @@ nv50_backlight_init(struct nouveau_backlight *bl,
 	struct nouveau_drm *drm = nouveau_drm(nv_encoder->base.base.dev);
 	struct nvif_object *device = &drm->client.device.object;
 
-	if (!nvif_rd32(device, NV50_PDISP_SOR_PWM_CTL(ffs(nv_encoder->dcb->or) -
1)))
+	if (!nvif_rd32(device, NV50_PDISP_SOR_PWM_CTL(ffs(nv_encoder->dcb->or) -
1)) ||
+	    nv_conn->base.status != connector_status_connected)
 		return -ENODEV;
 
 	if (nv_conn->type == DCB_CONNECTOR_eDP) {
-- 
2.34.1

On Fri, Feb 4, 2022 at 7:05 PM Lyude Paul <lyude at redhat.com>
wrote:
>
> It seems that some laptops will report having both an eDP and LVDS
> connector, even though only the LVDS connector is actually hooked up. This
> can lead to issues with backlight registration if the eDP connector ends up
> getting registered before the LVDS connector, as the backlight device will
> then be registered to the eDP connector instead of the LVDS connector.
>
> So, fix this by only registering the backlight on connectors that are
> reported as being connected.
>
I think the patch might risk breaking muxed systems where we have two
GPUs, but.. the systems I know of have different ways of controlling
the backlight anyway. So unless there is something I missed this is

Reviewed-by: Karol Herbst <kherbst at redhat.com>
> Signed-off-by: Lyude Paul <lyude at redhat.com>
> Fixes: 6eca310e8924 ("drm/nouveau/kms/nv50-: Add basic DPCD backlight
support for nouveau")
> Bugzilla: https://gitlab.freedesktop.org/drm/nouveau/-/issues/137
> Cc: <stable at vger.kernel.org> # v5.15+
> ---
>  drivers/gpu/drm/nouveau/nouveau_backlight.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> index ae2f2abc8f5a..6af12dc99d7f 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> @@ -294,7 +294,8 @@ nv50_backlight_init(struct nouveau_backlight *bl,
>         struct nouveau_drm *drm =
nouveau_drm(nv_encoder->base.base.dev);
>         struct nvif_object *device = &drm->client.device.object;
>
> -       if (!nvif_rd32(device,
NV50_PDISP_SOR_PWM_CTL(ffs(nv_encoder->dcb->or) - 1)))
> +       if (!nvif_rd32(device,
NV50_PDISP_SOR_PWM_CTL(ffs(nv_encoder->dcb->or) - 1)) ||
> +           nv_conn->base.status != connector_status_connected)
>                 return -ENODEV;
>
>         if (nv_conn->type == DCB_CONNECTOR_eDP) {
> --
> 2.34.1
>