Re: [PATCH] media: bcm2835-unicam: Remove RGB24 support

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Laurent, others,

On Tue, Jun 10, 2025 at 12:52:38PM +0300, Laurent Pinchart wrote:
> On Mon, Jun 09, 2025 at 03:33:29PM +0100, Dave Stevenson wrote:
> > On Mon, 9 Jun 2025 at 01:38, Laurent Pinchart wrote:
> > > On Fri, Jun 06, 2025 at 04:53:03PM +0200, Maxime Ripard wrote:
> > > > The RGB24 V4L2 format is defined as a pixel format with 8 bits per
> > > > components, with the components being in the red, green, and blue order
> > > > from left to right.
> > > >
> > > > The RGB MIPI-CSI2 is defined in the specification (Section 11.3.1,
> > > > RGB888) with blue coming first, then green, then red. So the opposite of
> > > > what V4L2 means by RGB.
> > > >
> > > > Since the hardware cannot reorder the components, this means that when
> > > > selecting the RGB24 format, you get inverted red and blue components
> > > > compared to what you'd expect.
> > > >
> > > > The driver already supports BGR24, so we can simply remove the RGB24
> > > > format from the driver.
> > >
> > > The only reason I could think of to explain why the driver exposes
> > > V4L2_PIX_FMT_RGB24 is to support CSI-2 sources that transfer RGB888 data
> > > with a non-standard order. I don't know what hardware would do that.
> > > Dave, Naush, do you recall why this pixel format is supported by the
> > > unicam driver ?
> > 
> > I've lost track of exactly what gets validated along the pipeline.
> > unicam_video_link_validate [1] looks to ensure that the V4L2 pixel
> > format and media bus codes match as listed in the table.
> > 
> > tc358743, adv7604, adv7511, and adv748x are all saying they produce
> > MEDIA_BUS_FMT_RGB888_1X24
> > ov5640 says it produces MEDIA_BUS_FMT_BGR888_1X24.
> > 
> > Is that an error in the ov5640 driver? If not, then both entries have
> > to be in the table to support all those drivers.
> 
> We haven't standardized which media bus code to use for 24-bit RGB on
> CSI-2, so I'm not surprised drivers didn't get it right :-( The adv7604
> and adv7511 shouldn't be relevant here though, as they're not CSI-2
> devices. tc358743 and adv748x are relevant.

My understanding is the order indeed is BGR. But let's say that if there's
a device that generates RGB (as in pixel order) pixel data, then it should
be reflected in V4L2 pixelformat as well.

> 
> Sakari, which media bus code would you expect ?

MEDIA_BUS_FMT_BGR888_1X24. That's actually used in the example as well:
<URL:https://hverkuil.home.xs4all.nl/spec/userspace-api/v4l/subdev-formats.html#v4l2-mbus-pixelcode>.

> 
> > Looking at alvium-csi2.c and st-mipid02.c, both mappings are included
> > there (and RBG888 in the case of alvium).
> > Unicam's hardware doesn't care about the ordering as it just writes
> > the incoming data to memory, so having all the sensible mappings
> > between MEDIA_BUS_FMT_* and V4L2_PIX_FMT_* values makes sense.
> > 
> > My initial reaction though is that simply removing the entry won't
> > solve the problem anyway. You won't get a match between the
> > MEDIA_BUS_FMT_RGB888_1X24 requested by tc358743 and a supported V4L2
> > pixel format, so the link_validate will fail.
> > Swapping either fourcc or code between the two entries would be the
> > fix I was expecting.
> > 
> > [1] https://github.com/torvalds/linux/blob/master/drivers/media/platform/broadcom/bcm2835-unicam.c#L2151-L2169
> > 
> > > > Fixes: 392cd78d495f ("media: bcm2835-unicam: Add support for CCP2/CSI2 camera interface")
> > > > Signed-off-by: Maxime Ripard <mripard@kernel.org>
> > > > ---
> > > >  drivers/media/platform/broadcom/bcm2835-unicam.c | 5 -----
> > > >  1 file changed, 5 deletions(-)
> > > >
> > > > diff --git a/drivers/media/platform/broadcom/bcm2835-unicam.c b/drivers/media/platform/broadcom/bcm2835-unicam.c
> > > > index f10064107d543caf867249d0566a0f42d6d8c4c6..1f549019efd53c9aae83193e74f1a3601ebf274d 100644
> > > > --- a/drivers/media/platform/broadcom/bcm2835-unicam.c
> > > > +++ b/drivers/media/platform/broadcom/bcm2835-unicam.c
> > > > @@ -338,15 +338,10 @@ static const struct unicam_format_info unicam_image_formats[] = {
> > > >       /* RGB Formats */
> > > >               .fourcc         = V4L2_PIX_FMT_RGB565, /* gggbbbbb rrrrrggg */
> > > >               .code           = MEDIA_BUS_FMT_RGB565_1X16,
> > > >               .depth          = 16,
> > > >               .csi_dt         = MIPI_CSI2_DT_RGB565,
> > > > -     }, {
> > > > -             .fourcc         = V4L2_PIX_FMT_RGB24, /* rgb */
> > > > -             .code           = MEDIA_BUS_FMT_RGB888_1X24,
> > > > -             .depth          = 24,
> > > > -             .csi_dt         = MIPI_CSI2_DT_RGB888,
> > > >       }, {
> > > >               .fourcc         = V4L2_PIX_FMT_BGR24, /* bgr */
> > > >               .code           = MEDIA_BUS_FMT_BGR888_1X24,
> > > >               .depth          = 24,
> > > >               .csi_dt         = MIPI_CSI2_DT_RGB888,
> > > >
> > > > ---
> > > > base-commit: 0ff41df1cb268fc69e703a08a57ee14ae967d0ca
> > > > change-id: 20250606-rpi-unicam-rgb-bgr-fix-d1b6f46a75ad
> 

-- 
Regards,

Sakari Ailus