[libcamera-devel] [PATCH] libcamera: pipeline: uvcvideo: Infer camera location from removable attribute

[Kieran Bingham]

Quoting Laurent Pinchart via libcamera-devel (2022-05-26 14:30:25)
> Use the removable attribute exposed by the USB device in sysfs to infer
> if the camera is an external (when removable) or front (when
> non-removable) camera. This is likely not perfect as the removable
> attribute is derived from ACPI data which may not always be accurate,
> and non-ACPI platforms likely report all devices as removable. Still,
> this is a first step upon which a better method could be built.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  src/libcamera/pipeline/uvcvideo/uvcvideo.cpp | 33 +++++++++++++++++---
>  1 file changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
> index 2ebf2788c3b4..53b2f23ab029 100644
> --- a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
> +++ b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
> @@ -501,13 +501,36 @@ int UVCCameraData::init(MediaDevice *media)
>  
>         video_->bufferReady.connect(this, &UVCCameraData::bufferReady);
>  
> -       /*
> -        * \todo Find a way to tell internal and external UVC cameras apart.
> -        * Until then, treat all UVC cameras as external.
> -        */
> -       properties_.set(properties::Location, properties::CameraLocationExternal);
>         properties_.set(properties::Model, utils::toAscii(media->model()));
>  
> +       /*
> +        * Derive the location from the device removable attribute in sysfs.
> +        * Non-removable devices are assumed to be front as we lack detailed
> +        * location information, and removable device are considered external.
> +        *
> +        * The sysfs removable attribute is derived from the ACPI _UPC attribute
> +        * if available, or from the USB hub descriptors otherwise. ACPI data
> +        * may not be very reliable, and the USB hub descriptors may not be
> +        * accurate on DT-based platforms. A heuristic may need to be
> +        * implemented later if too many devices end up being miscategorized.
> +        *
> +        * \todo Find a way to tell front and back devices apart. This could
> +        * come from the ACPI _PLD, but that may be even more unreliable than
> +        * the _UPC.
> +        */
> +       properties::LocationEnum location = properties::CameraLocationExternal;
> +       std::ifstream file(video_->devicePath() + "/../removable");

I expected this devicePath() to be /dev/video0 ... but given the above,
I presume it's mapping elsewhere. Is it guaranteed to map to the sysfs
path? or do we need to build it with the information we have?

Anyway, if it fails, it will fail to what we have anyway so - if it
works in some cases, that's still 'progress'...

With the path understood, (which I'm sure you do anyway...)

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> +       if (file.is_open()) {
> +               std::string value;
> +               std::getline(file, value);
> +               file.close();
> +
> +               if (value == "fixed")
> +                       location = properties::CameraLocationFront;
> +       }
> +
> +       properties_.set(properties::Location, location);
> +
>         /*
>          * Get the current format in order to initialize the sensor array
>          * properties.
> 
> base-commit: f8e4649680303ce9b7dace5ad1344fcb450013ac
> prerequisite-patch-id: edbf31ed9d7f5374e17b34936fe1aa8dea2eac44
> -- 
> Regards,
> 
> Laurent Pinchart
>