Re: [PATCH 2/2] media: uvcvideo: skip resume when writing hibernation image

[Laurent Pinchart]

On Tue, Apr 28, 2026 at 04:05:13PM +0800, Haowen Tu wrote:
> When a UVC camera is in active use and the system enters S4 hibernation,
> the camera is suspended as part of the normal device freeze sequence.
> However, after create_image() saves the memory snapshot, the kernel
> briefly resumes all devices with PMSG_THAW to write the hibernation image
> to storage.  This causes uvc_video_resume() to run and reinitialize the
> camera hardware, which visibly turns on the camera indicator LED during
> this intermediate phase -- even though the system is about to power off.
> 
> The UVC device is not needed during the image-write window, where the
> system only needs devices required for writing the hibernation image.
> USB .resume callbacks do not receive pm_message_t (unlike .suspend),
> so use the PM-layer helper to detect this phase and return early from
> uvc_video_resume(), preventing the unnecessary hardware reinitialization
> and the spurious LED activation.
> 
> Skipping the THAW resume is safe: stream->frozen remains 1 (set during
> the earlier FREEZE suspend), the device is powered off immediately after
> swsusp_write() with no intervening suspend, and the subsequent
> PMSG_RESTORE resume on the restored kernel calls uvc_video_resume()
> with pm_hibernation_storing_image() returning false, performing the full
> reinitialization as normal.
> 
> Signed-off-by: Haowen Tu <tuhaowen@xxxxxxxxxxxxx>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index f6c8e3223796..16a911b684d5 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -12,6 +12,7 @@
>  #include <linux/list.h>
>  #include <linux/module.h>
>  #include <linux/slab.h>
> +#include <linux/suspend.h>
>  #include <linux/usb.h>
>  #include <linux/usb/hcd.h>
>  #include <linux/videodev2.h>
> @@ -2135,6 +2136,15 @@ int uvc_video_resume(struct uvc_streaming *stream, int reset)
>  {
>  	int ret;
>  
> +	/*
> +	 * After taking the hibernation memory snapshot, the kernel briefly resumes
> +	 * devices with PMSG_THAW to write the image to storage before powerdown.
> +	 * The UVC device is not involved in storage I/O, so skip reinitializing
> +	 * it to avoid unnecessary USB traffic during this transient phase.
> +	 */

Will all leaf drivers need to implement something similar ? How does
that scale ?

> +	if (pm_hibernation_storing_image())
> +		return 0;
> +
>  	/*
>  	 * If the bus has been reset on resume, set the alternate setting to 0.
>  	 * This should be the default value, but some devices crash or otherwise

-- 
Regards,

Laurent Pinchart