linux-kernel - Re: [PATCH v3 00/24] i.MX Media Driver

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Date:   Tue, 31 Jan 2017 11:09:24 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Steve Longerbeam <slongerbeam@...il.com>,
        Hans Verkuil <hverkuil@...all.nl>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, mchehab@...nel.org, nick@...anahar.org,
        markus.heiser@...marIT.de,
        laurent.pinchart+renesas@...asonboard.com, bparrot@...com,
        geert@...ux-m68k.org, arnd@...db.de, sudipm.mukherjee@...il.com,
        minghsiu.tsai@...iatek.com, tiffany.lin@...iatek.com,
        jean-christophe.trotin@...com, horms+renesas@...ge.net.au,
        niklas.soderlund+renesas@...natech.se, robert.jarzmik@...e.fr,
        songjun.wu@...rochip.com, andrew-ct.chen@...iatek.com,
        gregkh@...uxfoundation.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-media@...r.kernel.org, devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v3 00/24] i.MX Media Driver

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