On 4/16/19 3:54 PM, Rodin, Michael (Ferchau; ADITG/ESM1) wrote: > 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. The spec is a bit out of date: those missing field values were probably added after this text was written. I'll make a patch fixing this. > > 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. IMHO the text is reasonably clear on that. But if you have suggestions to improve it, then make a proposal. > 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." Whoops, that makes no sense. There are no drivers that do this. I'll remove this line. If a driver can't tell the difference, then it should just pick FIELD_TOP or 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. Actually, drivers cannot ever return NONE for a top or bottom field. FIELD_NONE indicates that a full frame has arrived, and doing something else would break userspace. > > 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? It means that the source video transmitted full frames, not top and bottom fields. I clarified the text a bit. > 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)? For video capture (that's what we are talking about here) it is returned by the driver in v4l2_buffer, never by userspace. Userspace can try to request a specific field value when calling S_FMT, but the driver can overwrite it. The possible field values that a driver can support are dependent on the video source (i.e. sensors are always FIELD_NONE) and the hardware capabilities. > 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. Certainly not. FIELD_INTERLACED combines two fields into a single buffer, but the odd and even lines in the frame were captured at different times. Whereas for FIELD_NONE all lines were captured at the same time. So a FIELD_INTERLACED buffer may need to undergo additional deinterlacing. If the hardware already does high-quality deinterlacing then that might be a reason for the driver to return FIELD_NONE to avoid additional deinterlacing in userspace. In practice there are three main categories in the way the field is handled: 1) The video source is a webcam: field is always FIELD_NONE, set by the driver. 2) The video source is HDMI: if the video is progressive, then the field is always FIELD_NONE. If the video is interlaced, then the field is always FIELD_ALTERNATE in v4l2_format and alternating FIELD_TOP and BOTTOM in v4l2_buffer. 3) The video source is SDTV (i.e. S-Video or composite): the video is always interlaced, and it depends on the hardware which field values are supported, except for FIELD_NONE, which is never returned as far as I am aware. Regards, Hans > > 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 >
