Hello, Trying to capture images in JPEG format with regular "image sensor -> mipi-csi receiver -> host interface" H/W configuration I've found there is no standard way to communicate between the sensor subdev and the host driver what is exactly a required maximum buffer size to capture a frame.
For the raw formats there is no issue as the buffer size can be easily determined from the pixel format and resolution (or sizeimage set on a video node). However compressed data formats are a bit more complicated, the required memory buffer size depends on multiple factors, like compression ratio, exact file header structure etc. Often it is at the sensor driver where all information required to determine size of the allocated memory is present. Bridge/host devices just do plain DMA without caring much what is transferred. I know of hardware which, for some pixel formats, once data capture is started, writes to memory whatever amount of data the sensor is transmitting, without any means to interrupt on the host side. So it is critical to assure the buffer allocation is done right, according to the sensor requirements, to avoid buffer overrun. Here is a link to somehow related discussion I could find: [1] http://www.mail-archive.com/linux-media@vger.kernel.org/msg27138.html In order to let the host drivers query or configure subdevs with required frame buffer size one of the following changes could be done at V4L2 API: 1. Add a 'sizeimage' field in struct v4l2_mbus_framefmt and make subdev drivers optionally set/adjust it when setting or getting the format with set_fmt/get_fmt pad level ops (and s/g_mbus_fmt ?) There could be two situations: - effective required frame buffer size is specified by the sensor and the host driver relies on that value when allocating a buffer; - the host driver forces some arbitrary buffer size and the sensor performs any required action to limit transmitted amount of data to that amount of data; Both cases could be covered similarly as it's done with VIDIOC_S_FMT. Introducing 'sizeimage' field is making the media bus format struct looking more similar to struct v4l2_pix_format and not quite in line with media bus format meaning, i.e. describing data on a physical bus, not in the memory. The other option I can think of is to create separate subdev video ops. 2. Add new s/g_sizeimage subdev video operations The best would be to make this an optional callback, not sure if it makes sense though. It has an advantage of not polluting the user space API. Although 'sizeimage' in user space might be useful for some purposes I rather tried to focus on "in-kernel" calls. Comments? Better ideas? Thanks, -- Sylwester Nawrocki Samsung Poland R&D Center -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
