On Mon, 2017-01-30 at 13:06 +0000, Russell King - ARM Linux wrote: > > The central issue seems to be that I think media pad links / media bus > > formats should describe physical links, such as parallel or serial > > buses, and the formats of pixels flowing through them, whereas Steve > > would like to extend them to describe software transports and in-memory > > formats. > > This probably isn't the right place to attach this comment in this > thread, but... the issue of media bus formats matching physical buses > is an argument that I think is already lost. It is unfortunate that the parallel format definitions have been reused for the MIPI logical formats, but I suppose we have to live with that. Still, I think this is not a reason to open the floodgates and start describing in-memory formats using MEDIA_BUS_FMT_* Does at least the combination of logical format and number of lanes uniquiely describe the physical format? For the 4-lane LVDS bus formats there are JEIDA/VESA variants where just the bit ordering is different (MEDIA_BUS_FMT_RGB888_1X7X4_SPWG, MEDIA_BUS_FMT_RGB888_1X7X4_JEIDA). > For example, take the 10-bit bayer formats: > > #define MEDIA_BUS_FMT_SBGGR10_1X10 0x3007 > #define MEDIA_BUS_FMT_SGBRG10_1X10 0x300e > #define MEDIA_BUS_FMT_SGRBG10_1X10 0x300a > #define MEDIA_BUS_FMT_SRGGB10_1X10 0x300f > > These are commonly used on CSI serial buses (see the smiapp driver for > example). From the description at the top of the file, it says the > 1X10 means that one pixel is transferred as one 10-bit sample. > > However, the format on wire is somewhat different - four pixels are > transmitted over five bytes: > > P0 P1 P2 P3 P0 P1 P2 P3 > 8-bit 8-bit 8-bit 8-bit 2-bit 2-bit 2-bit 2-bit > > This gives two problems: > 1) it doesn't fit in any sensible kind of "one pixel transferred as > N M-bit samples" description because the pixel/sample values > (depending how you look at them) are broken up. > > 2) changing this will probably be a user visible change, as things > like smiapp are already in use. > > So, I think what we actually have is the media bus formats describing > the _logical_ bus format. Yes, one pixel is transferred as one 10-bit > sample in this case. Yes. I suppose one way to look at it is that the MIPI CSI-2 specified formats are representations of corresponding parallel bus formats. > To help illustrate my point, consider the difference between > MEDIA_BUS_FMT_RGB565_1X16 and MEDIA_BUS_FMT_RGB565_2X8_BE or > MEDIA_BUS_FMT_RGB565_2X8_LE. RGB565_1X16 means 1 pixel over an effective > 16-bit wide bus (if it's not 16-bit, then it has to be broken up into > separate "samples".) RGB565_2X8 means 1 pixel as two 8-bit samples. > > So, the 10-bit bayer is 1 pixel as 1.25 bytes. Or is it, over a serial > bus. Using the RGB565 case, 10-bit bayer over a 4 lane CSI bus becomes > interesting: > > first byte 2nd 3rd > lane 1 P0 9:2 S0 P7 9:2 > lane 2 P1 9:2 P4 9:2 S1 > lane 3 P2 9:2 P5 9:2 P8 9:2 > lane 4 P3 9:2 P6 9:2 P9 9:2 > > S0 = P0/P1/P2/P3 least significant two bits > S1 = P4/P5/P6/P7 least significant two bits > > or 2 lane CSI: > first byte 2nd 3rd 4th 5th > lane 1 P0 9:2 P2 S0 P5 P7 > lane 2 P1 9:2 P3 P4 P6 S1 > > or 1 lane CSI: > lane 1 P0 P1 P2 P3 S0 P4 P5 P6 P7 S1 P8 P9 ... These do look like three different media bus formats to me. regards Philipp -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
