libcamera: v4l2_pixelformat: Add helper function to get the description

Add a helper function to V4L2PixelFormat for retrieving the V4L2
description string.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>

include/libcamera/internal/v4l2_pixelformat.h

@@ -40,6 +40,7 @@ public:
 	operator uint32_t() const { return fourcc_; }
 
 	std::string toString() const;
+	const char *description() const;
 
 	PixelFormat toPixelFormat() const;
 	static V4L2PixelFormat fromPixelFormat(const PixelFormat &pixelFormat,

src/libcamera/v4l2_pixelformat.cpp

@@ -250,6 +250,28 @@ std::string V4L2PixelFormat::toString() const
 	return ss;
 }
 
+/**
+ * \brief Retrieve the V4L2 description for the format
+ *
+ * The description matches the value used by the kernel, as would be reported
+ * by the VIDIOC_ENUM_FMT ioctl.
+ *
+ * \return The V4L2 description corresponding to the V4L2 format, or a
+ * placeholder description if not found
+ */
+const char *V4L2PixelFormat::description() const
+{
+	const auto iter = vpf2pf.find(*this);
+	if (iter == vpf2pf.end()) {
+		LOG(V4L2, Warning)
+			<< "Unsupported V4L2 pixel format "
+			<< toString();
+		return "Unsupported format";
+	}
+
+	return iter->second.description;
+}
+
 /**
  * \brief Convert the V4L2 pixel format to the corresponding PixelFormat
  * \return The PixelFormat corresponding to the V4L2 pixel format