Hi, I would like to ask several questions regarding the documentation of the enum "v4l2_field" [1]. These questions came up during my investigations of issues with interaction between the gstreamer plugin v4l2src and the rcar video input driver [2]. The documentation [1] specifies that: "All video capture and output devices must report the current field order. Some drivers may permit the selection of a different order, to this end applications initialize the field field of struct v4l2_pix_format before calling the VIDIOC_S_FMT ioctl. If this is not desired it should have the value V4L2_FIELD_ANY (0)." If I have understood these lines correctly, this means that if userspace sets "field" member of the struct "v4l2_pix_format" to V4L2_FIELD_ANY and uses this as parameter for the VIDIOC_S_FMT ioctl, then a driver should select/report the field order, which was previously set by media-ctl utility in the next subdevice, which is connected to the /dev/videoX node (From my understanding this would be equivalent to the "current field order"). If the described behavior is correct, then the description in the table row for V4L2_FIELD_ANY in [1] is incomplete: "Applications request this field order when any one of the V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, or V4L2_FIELD_INTERLACED formats is acceptable." What if V4L2_FIELD_ALTERNATE or V4L2_FIELD_SEQ_TB or V4L2_FIELD_SEQ_BT are also acceptable for the application? I think that the specification is either unprecise or my understanding of the specification is wrong. Another potential issue, which I found in this documentation is that it does not distinguish between multiple contexts in which enum v4l2_field can be used. I can think of at least two different contexts: - When used to select the field order with VIDIOC_S_FMT ioctl. - When used to report the field order in a buffer: for example application sets V4L2_FIELD_ALTERNATE in VIDIOC_S_FMT ioctl and then gets buffers, which have V4L2_FIELD_TOP/BOTTOM set. Now with this in mind, when I read the description of V4L2_FIELD_NONE: "The driver may also indicate this order when it cannot distinguish between V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM." I see two possible meanings/interpretations: - If application sets V4L2_FIELD_ALTERNATE in VIDIOC_S_FMT ioctl, report V4L2_FIELD_NONE back so the application knows that the driver can not provide any TOP/BOTTOM metadata in the buffers (which may be necessary for the application for example for deinterlacing) before it has got any buffer. - If application sets V4L2_FIELD_ALTERNATE in VIDIOC_S_FMT ioctl, driver reports V4L2_FIELD_ALTERNATE back, even if it can not distinguish between TOP/BOTTOM. But when the application starts to read buffers, they have V4L2_FIELD_NONE set if it's not possible to distinguish between TOP/BOTTOM. Also there is another ambiguity in the description of V4L2_FIELD_NONE: "Images are in progressive format, not interlaced." What does "interlaced" mean in this case? Does it mean the other possible enum values or just the V4L2_FIELD_INTERLACED? If this just means V4L2_FIELD_INTERLACED, then it would imply that for example V4L2_FIELD_SEQ_TB and V4L2_FIELD_ALTERNATE are progressive formats, which is obviously not true. And also generally, in which of described contexts should be V4L2_FIELD_NONE set or reported (buffer or VIDIOC_S_FMT ioctl)? Another point is that V4L2_FIELD_INTERLACED is also used by v4l2src to tell rcar-vin driver to combine the fields before giving them to application, so basically it requests progressive signal. So the meanings of V4L2_FIELD_INTERLACED and V4L2_FIELD_NONE are basically the same in this case. Thank you in advance and sorry for the long mail! [1] Documentation/media/uapi/v4l/field-order.rst [2] drivers/media/platform/rcar-vin Best regards Michael Rodin Advanced Driver Information Technology GmbH Engineering Software Multimedia 1 (ADITG/ESM1) Robert-Bosch-Str. 200 31139 Hildesheim Germany Tel. +49 5121 49 6936 Fax +49 5121 49 6999 mrodin@xxxxxxxxxxxxxx Web: www.adit-jv.com ADIT is a joint venture company of Robert Bosch GmbH/Robert Bosch Car Multimedia GmbH and DENSO Corporation Sitz: Hildesheim, Registergericht: Amtsgericht Hildesheim HRB 3438 Geschaeftsfuehrung: Wilhelm Grabow, Ken Yaguchi
