Hi Tomi, On Wed, Sep 06, 2023 at 11:28:37AM +0300, Tomi Valkeinen wrote: > On 05/09/2023 19:38, Laurent Pinchart wrote: > > On Thu, Aug 24, 2023 at 11:26:56AM +0300, Tomi Valkeinen wrote: > >> On 24/08/2023 10:24, Sakari Ailus wrote: > >>> On Wed, Aug 23, 2023 at 04:16:13PM +0300, Tomi Valkeinen wrote: > >>>> On 08/08/2023 10:55, Sakari Ailus wrote: > >>>>> Add generic serial metadata mbus formats. These formats describe data > >>>>> width and packing but not the content itself. The reason for specifying > >>>>> such formats is that the formats as such are fairly device specific but > >>>>> they are still handled by CSI-2 receiver drivers that should not be aware > > > > Do we want these formats to be CSI-2-specific ? > > > >>>>> of device specific formats. What makes generic metadata formats possible > >>>>> is that these formats are parsed by software only, after capturing the > >>>>> data to system memory. > >>>> > >>>> If I'm not mistaken, the CSI-2 spec doesn't say much about embedded data, > >>>> except that it may exist. Afaics, in CSI-2, the embedded data is split into > >>>> "lines", although the amount of data can be different than in the video > >>>> lines. > >>>> > >>>> The CCS specs talks more about embedded data. Some of it is quite odd, like: > >>>> "The length of the embedded data line shall not exceed the length of the > >>>> image data line. The embedded data line should have the same length as the > >>>> image data line.". I think it means the embedded line can be shorter than > >>>> image line, but not longer. > >>> > >>> That's what it means, yes. The CCS also has means to obtain the actual > >>> length --- frame format descriptors. > >>> > >>>> CCS also says that an embedded line should use 0x07 as padding at the end of > >>>> the line, if there's less data than the image line. > >>>> > >>>> CCS also talks about how the embedded data would be packed, and in some > >>>> cases that packing would be the same as for pixel data. > >>> > >>> In fact the packing is the same as for pixel data: the CSI-2 does not > >>> really differentiate there. > >> > >> If I understand the CCS spec right, the packing is not the same as for > >> the pixel data. You can have RAW12 pixel data but 8-byte embedded data. > >> But if you meant that the different packing style options are the same > >> for pixel and embedded data, yes. > > > > The idea is to allow CSI-2 receivers to unpack embedded data the same > > way as pixel data at the hardware level, and guarantee that no embedded > > Yes, but my comment was to Sakari's "the packing is the same as for > pixel data" comment. Afaics, there's nothing in CSI-2 or CCS that says > the packing is the same for pixel data and embedded data. > > In fact, the CSS says "By default, top-embedded data shall use the same > data packing as the Visible pixel data with one embedded data byte per > pixel", implying that it would not always be the case. It continues that > for RAW16/RAW20/RAW24 pixels the embedded data could be sent with > RAW8/RAW10/RAW12. The spec if a bit ambiguous :-S "by default" implies that something else could be done, but it's not clear what the other options are. > That was just for top-embedded data, and I couldn't right away see > mention of other embedded data. The specification also mentions PDAF data for instance. > > data significant byte gets split across multiple samples. When unpacking > > RAW10 pixel data, the CSI-2 receiver will take 5 bytes and output 4 > > samples on a 10-bit wide bus. For embedded data, it can conceptually do > > that same, and output embedded data with one byte per sample, aligned on > > the MSB of the 10-bit bus. However, in practice, I think the CSI-2 > > receiver will simply receive the bytes and send them to the DMA engine, > > without unpacking and then repacking. > > At least TI CAL CSI-2 RX can extract the data from the CSI-2 stream > using different unpacking for pixel and embedded data, and then > (optionally) pack it in different ways when writing to memory. Does this mean that the CAL could take RAW10 embedded data and drop the padding bytes when writing to memory ? > > This can also be viewed as away to allow CSI-2 transmitters to serialize > > pixel data and embedded data the same way. For a RAW10 sensor, for > > instance, the embedded data would be generated on an internal 10-bit > > wide bus, aligned to the MSB, and would go through CSI-2 packing the > > same way that pixels do. That's nice, but it's internal to the sensor, > > so we don't need to expose it to userspace on that side. > > > >>>> But I don't think these formats are generic. They're defined in CCS, so > >>>> shouldn't the format be, e.g., MEDIA_BUS_FMT_META_CCS_RAW_12 or such? > >>> > >>> The reason for having generic definitions is that we do not need receiver > >>> drivers to be aware of formats that are specific to another driver. > >> > >> Yes, I agree with that, and that's not my point here. But "generic" > >> doesn't mean the definitions are not for some particular bus or > >> standard, "generic" just means that it doesn't specify the content, only > >> the packing. > >> > >> My point is that these packings seem to be specific to CCS. While > >> non-CCS compliant sensors may use the same packing, the packing itself > >> is still a "CCS packing". > > > > Agreed. > > > >> So why not call them that? The 8-bit format is > >> fully generic, whereas the rest are CCS packings, as defined in the CCS > >> spec (the CCS spec also specifies the content, but here were only using > >> the CCS packing). > > > > I think an additional question is whether we actually need multiple > > media bus codes here. Are there CSI-2 receivers, or other downstream > > blocks, that will need to be configured differently to receive embedded > > data from a CCS (in the sense of using CCS packing, not being fully > > CCS-compliant) RAW10 sensor compared to a RAW12 sensor ? Or will the > > CSI-2 receiver just send bytes to memory without caring ? In the latter > > case, it will be the application that will need to know how the data is > > packed to ignore one byte out of three for RAW12 and out of five for > > RAW10. > > I would presume all receivers can operate in "just write what you > receive" mode, in which case the packing doesn't matter as you say, but > then again, I think it's safer to have mbus codes for the different > packings. > > >> Maybe they shouldn't be called "generic", but... umm... Content unaware > >> metadata formats... doesn't sound very good =). > >> > >> Also, I'm a bit confused about CSI-2 pixel and embedded data formats and > >> how we handle them. > >> > >> For MEDIA_BUS_FMT_SBGGR10_1X10, we define that the data contains 10 bits > >> per pixel, from bit 0 to 9. But CSI-2, for RAW10, actually sends it > >> differently, with the higher bits first, and the lowest bits in the > >> fourth byte. So that CSI-2 packing is implicit, kind of hidden here. > > > > Usage of MEDIA_BUS_FMT_SBGGR10_1X10 for CSI-2 is a historical decision > > (or mistake, depending on how you look at it). It doesn't match what > > actually happens on the bus. > > > >> Which is probably fine, as we're really only interested in the unpacked > >> data, not the CSI-2 packed. And when writing this data to memory, the > >> DMA engine can write it either as is, or unpack each pixel to a 16-bit > >> container. > >> > >> For MEDIA_BUS_FMT_META_10, we define it similarly to > >> MEDIA_BUS_FMT_SBGGR10_1X10, except the lowest 2 bits are marked as > >> padding bits. And, I think, MEDIA_BUS_FMT_META_10 implies RAW10 CSI-2 > >> format. > > > > That's how I understand it too > > > >> However, when writing the data to memory, we don't want either > >> of the modes used in the above pixel data case, but rather we want to > >> write the data as it is in the CSI-2 bus. So, the DMA engine can either > >> reverse the RAW10 unpacking to get the wanted output format, or > >> alternatively the CSI-2 receiver could instead use RAW8 mode to avoid > >> any unpacking. > > > > I don't think the DMA engine would "reverse the RAW10 packing" here. It > > will write bytes as they arrive (or more likely, a set of bytes at a > > time to operate on bursts). The other option would be for the DMA engine > > to drop the padding bytes, but I don't know of any CSI-2 receiver that > > can do that. > > Well, for pixel data, TI CAL has the following options when writing to > memory for, e.g. RAW12: > - RAW12 MIPI (a separate byte for lsbs) > - RAW12 Linear (12 consecutive bits per pixel) > - RAW16 (12 consecutive bits per pixel in a 16bit container) > > Of these, only the RAW12 MIPI makes sense for embedded data (actually > the whole processing block can be and should be skipped for embedded > data, so it's more like CAL is receiving RAW8 and writing out RAW8). > > Or, if it would be possible, skipping the padding bytes. Which looks > like it is almost possible with TI CAL, but not quite: it can write with > RAW8, but afaics that always takes the lowest bits of the 12-bit pixel, > but here we'd like it to take the highest bits. That's a weird one, even for image data, dropping the MSBs doesn't make much sense, while writing the RAW8 MSB when receiving RAW10 or RAW12 would be useful. > Anyway, I don't think there's any issue here, I was just writing down my > thoughts. As long as we have good mbus formats which describe the > packing fully, and V4L2 formats which describe the memory formats, we > should be fine. > > >> Does the above make any sense? I'm a bit confused about all the packings > >> and unpackings =). Does MEDIA_BUS_FMT_META_10 mean that the CSI-2 TX > >> uses RAW10 CSI-2 packing, but the receiver uses RAW8 CSI-2 packing? > >> > >>> If there is a need for new generic formats that do not match this, we can > >>> always add more. But the point is: drivers for devices that do not produce > >>> the data should never deal with (device) specific formats. > >>> > >>> What comes to CSI-2 and these formats --- on parallel buses you might have > >>> the data aligned to the least significant bits instead. But is there a need > >>> to transport such data on parallel buses, at least so it would be expressed > >>> in mbus formats? > >> > >> I have a parallel sensor that sends embedded data. I'll have a look how > >> it organizes the data. -- Regards, Laurent Pinchart
