hack: media: qcom: camss: fix sensor pad null-deref at boot

Due to an unresolved bug in camss (or v4l2), user space can access the
camera device before things have been fully set up.

This specifically results in a NULL pointer dereference in
camss_find_sensor_pad() when udev tries to identify the v4l2 device
during boot (and this in turn prevents the display from being probed).

    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000030

    CPU: 3 UID: 0 PID: 442 Comm: v4l_id Not tainted 6.15.0-rc1 torvalds#106 PREEMPT

    Hardware name: LENOVO 21BYZ9SRUS/21BYZ9SRUS, BIOS N3HET87W (1.59 ) 12/05/2023

    Call trace:
     camss_find_sensor_pad+0x20/0x74 [qcom_camss] (P)
     camss_get_pixel_clock+0x18/0x64 [qcom_camss]
     vfe_get+0xb8/0x504 [qcom_camss]
     vfe_set_power+0x30/0x58 [qcom_camss]
     pipeline_pm_power_one+0x13c/0x150 [videodev]
     pipeline_pm_power.part.0+0x58/0xf4 [videodev]
     v4l2_pipeline_pm_use+0x58/0x94 [videodev]
     v4l2_pipeline_pm_get+0x14/0x20 [videodev]
     video_open+0x78/0xf4 [qcom_camss]
     v4l2_open+0x80/0x120 [videodev]

Work around the bug by bailing out if camss_find_sensor_pad() is called
for an uninitialised media entity to allow machines like the Lenovo
ThinkPad X13s to boot.

Link: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Johan Hovold <[email protected]>

Author: jhovold

drivers/media/platform/qcom/camss/camss.c

@@ -2845,6 +2845,14 @@ struct media_pad *camss_find_sensor_pad(struct media_entity *entity)
 
 	while (1) {
 		pad = &entity->pads[0];
+
+		/*
+		 * Work around unresolved bug in camss (or v4l2) which can
+		 * result in pad being NULL here.
+		 */
+		if (WARN_ON(!pad))
+			return NULL;
+
 		if (!(pad->flags & MEDIA_PAD_FL_SINK))
 			return NULL;