On Fri, 6 Nov 2009, Hans Verkuil wrote: > On Thursday 05 November 2009 19:56:04 Guennadi Liakhovetski wrote: > > On Thu, 5 Nov 2009, Hans Verkuil wrote: > > > > > On Thursday 05 November 2009 17:51:50 Guennadi Liakhovetski wrote: > > > > On Thu, 5 Nov 2009, Hans Verkuil wrote: > > > > > > > > > On Friday 30 October 2009 15:01:27 Guennadi Liakhovetski wrote: > > > > > > Video subdevices, like cameras, decoders, connect to video bridges over > > > > > > specialised busses. Data is being transferred over these busses in various > > > > > > formats, which only loosely correspond to fourcc codes, describing how video > > > > > > data is stored in RAM. This is not a one-to-one correspondence, therefore we > > > > > > cannot use fourcc codes to configure subdevice output data formats. This patch > > > > > > adds codes for several such on-the-bus formats and an API, similar to the > > > > > > familiar .s_fmt(), .g_fmt(), .try_fmt(), .enum_fmt() API for configuring those > > > > > > codes. After all users of the old API in struct v4l2_subdev_video_ops are > > > > > > converted, the API will be removed. > > > > > > > > > > OK, this seems to completely disregard points raised in my earlier "bus and > > > > > data format negotiation" RFC which is available here once www.mail-archive.org > > > > > is working again: > > > > > > > > > > http://www.mail-archive.com/linux-media%40vger.kernel.org/msg09644.html > > > > > > > > > > BTW, ignore the 'Video timings' section of that RFC. That part is wrong. > > > > > > > > > > The big problem I have with this proposal is the unholy mixing of bus and > > > > > memory formatting. That should be completely separated. Only the bridge > > > > > knows how a bus format can be converted into which memory (pixel) formats. > > > > > > > > Please, explain why only the bridge knows about that. > > > > > > > > My model is the following: > > > > > > > > 1. we define various data formats on the bus. Each such format variation > > > > gets a unique identification. > > > > > > > > 2. given a data format ID the data format is perfectly defined. This > > > > means, you do not have to have a special knowledge about this specific > > > > format to be able to handle it in some _generic_ way. A typical such > > > > generic handling on a bridge is, for instance, copying the data into > > > > memory "one-to-one." For example, if a sensor delivers 10 bit monochrome > > > > data over an eight bit bus as follows > > > > > > > > y7 y6 y5 y4 y3 y2 y1 y0 xx xx xx xx xx xx y9 y8 ... > > > > > > > > then _any_ bridge, capable of just copying data from the bus bytewise into > > > > RAM will be able to produce little-endian 10-bit grey pixel format in RAM. > > > > This handling is _not_ bridge specific. This is what I call packing. > > > > > > Of course it is bridge dependent. It is the bridge that takes data from the > > > bus and puts it in memory. In many cases that is done very simply by bytewise > > > copying. Other bridges can do RGB to YUV or vice versa conversions or can do > > > endianness conversion or can do JPEG/MPEG compression on the fly or whatever > > > else hardware designers will think of. > > > > > > It's no doubt true for the SoCs you have been working with, but it is not so > > > simple in general. > > > > Ok, I forgot to mention one more point in the model: > > > > 4. Each bridge has _two_ ways to process data: data-format-specific and > > generic (pass-through). It's the _former_ one that is bridge specific, > > quite right! For a bridge to be able to process a data format, that it can > > process in a _special_ way, it doesn't need v4l2_imgbus_pixelfmt, it's > > only for data-formats, that bridges do _not_ know specifically they need > > it. In that _generic_ case it is not bridge-specific and a bridge driver > > can just look into the respective v4l2_imgbus_pixelfmt descriptor. > > > > Consider the following: a bridge can process N formats in a specific way. > > It knows which bits in which order represent which colours, etc. In such a > > case you just tell the driver "format X" and that's all it has to know > > about it to be able to handle it. > > > > The sensor, connected to the bridge, can also provide format Y, which the > > bridge doesn't know about. So what, there's then no way to use that > > format? Or do we have to add a _special_ handling rule for each format to > > each bridge driver?... > > > > > > 3. Therefore, each bridge, capable of handling of some "generic" data > > > > using some specific packing, can perfectly look through data-format > > > > descriptors, see if it finds any with the supported packing, and if so, it > > > > _then_ knows, that it can use that specific data format and the specific > > > > packing to produce the resulting pixel format from the format descriptor. > > > > > > > > > A bus format is also separate from the colorspace: that is an independent > > > > > piece of data. > > > > > > > > Sure. TBH, I do not quite how enum v4l2_colorspace is actually used. Is it > > > > uniquely defined by each pixel format? So, it can be derived from that? > > > > Then it is indeed redundant. Can drop, don't care about it that much. > > > > > > It's independent from the pixel format. So the same pixel (or bus) format can > > > have different colorspaces. > > > > Then I do not understand what a colourspace means in v4l context. You mean > > a yuv format can belong to a jpeg, or an srgb space?... > > No, it's not that extreme, but e.g. the same yuv format can be used with > different colorspaces depending on the source. I don't have the datasheet > handy but I know that for HDMI inputs there are different RGB colorspaces > depending on the input resolution. So while the dataformat is the same, > the colorspace will be different. Ok, so, you mean something like colour components are assigned in the same way in data tuples, but, for example, colour value ranges can be different? As for whether a colour-space field is needed in struct v4l2_imgbus_pixelfmt, actually, I think, it is. Otherwise how would you reply to G_FMT and TRY_FMT requests? > > > > > Personally I would just keep using v4l2_pix_format, except > > > > > that the fourcc field refers to a busimg format rather than a pixel format > > > > > in the case of subdevs. In most non-sensor drivers this field is completely > > > > > ignored anyway since the bus format is fixed. > > > > > > > > Example: there are cameras, that can be configured to pad 2 bits from the > > > > incomplete byte above to 10 either in high or in low bits. Do you want to > > > > introduce a new FOURCC code for those two formats? This is an example of > > > > what I call packing. > > > > > > If this happens in the sensor, then yes. > > > > No, those are two data formats, as produced by a camera sensor on the bus. > > What is made out of them in RAM is a completely separate issue. > > > > > > > I don't mind if you do a bus format to pixel format mapping inside soc-camera, > > > > > but it shouldn't spill over into the v4l core code. > > > > > > > > Don't understand. This is not for soc-camera only. This infrastructure > > > > should be used by all subdev drivers, communicating aver a data bus. The > > > > distinction is quite clear to me: if two entities connect over a bus, they > > > > use an image-bus data format to describe the data format. If they write > > > > and read from RAM - that's pixel format. > > > > > > We agree about that, but why then does struct v4l2_imgbus_framefmt contain > > > memory-related fields like packing, order and bits_per_sample? A subdev driver > > > does not care about that. All it has are X pins through which the data has to > > > pass. How that will look like in memory it doesn't know and doesn't care. > > > > That's right. subdev drivers do not care about v4l2_imgbus_framefmt, it's > > only bridge drivers, that do. > > I think most of my objections would probably go away if you redid your subdev > API so that the subdev only gets the data format and none of the packing data. > > The subdev API should not contain anything that it doesn't need. Otherwise it > becomes very confusing. Sorry, I grepped drivers and headers for v4l2_imgbus_pixelfmt and only see it used in host drivers. Can you point out more precisely what you mean? Thanks Guennadi --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
