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Message-ID: <CAPY8ntCYG8ufxpMkgBj1ZpSW-H2HObpcbQNg9tj+EXUM4PGkfQ@mail.gmail.com>
Date: Thu, 12 Jun 2025 18:01:40 +0100
From: Dave Stevenson <dave.stevenson@...pberrypi.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, Hans Verkuil <hverkuil@...all.nl>,
Mats Randgaard <matrandg@...co.com>, Alain Volmat <alain.volmat@...s.st.com>,
Sakari Ailus <sakari.ailus@...ux.intel.com>, linux-media@...r.kernel.org,
linux-kernel@...r.kernel.org, Hans Verkuil <hans.verkuil@...co.com>
Subject: Re: [PATCH 3/4] media: tc358743: Fix the RGB MBUS format
Hi Maxime
On Thu, 12 Jun 2025 at 13:54, Maxime Ripard <mripard@...nel.org> wrote:
>
> The tc358743 is an HDMI to MIPI-CSI2 bridge. It supports two of the
> three HDMI 1.4 video formats: RGB 4:4:4 and YCbCr 422.
>
> RGB 4:4:4 is converted to the MIPI-CSI2 RGB888 video format, and listed
> in the driver as MEDIA_BUS_FMT_RGB888_1X24.
>
> Most CSI2 receiver drivers then map MEDIA_BUS_FMT_RGB888_1X24 to
> V4L2_PIX_FMT_RGB24.
>
> However, V4L2_PIX_FMT_RGB24 is defined as having its color components in
> the R, G and B order, from left to right. MIPI-CSI2 however defines the
> RGB888 format with blue first.
>
> This essentially means that the R and B will be swapped compared to what
> V4L2_PIX_FMT_RGB24 defines.
>
> The proper MBUS format would be BGR888, so let's use that.
I know where you're coming from, but this driver has been in the tree
since 2015, so there is a reasonable expectation of users. I've had an
overlay for it in our kernel tree since 4.14 (July 2018), and I know
of at least PiKVM [1] as a product based on it. I don't know if Cisco
are still supporting devices with it in.
Whilst the pixel format may now be considered to be incorrect,
changing it will break userspace applications that have been using it
for those 10 years if they're explicitly looking for
MEDIA_BUS_FMT_RGB888_1X24 or the mapping of it through to
V4L2_PIX_FMT_RGB24.
The kernel docs at [2] quote Linus as saying
"If you break existing user space setups THAT IS A REGRESSION.
It's not ok to say "but we'll fix the user space setup"
Really. NOT OK."
I'm thinking of GStreamer if the format has been specified explicitly
- it'll fail to negotiate due to v4l2src saying it can't handle the
caps.
Yes it sucks as a situation, but I'm not sure what the best solution
is. Potentially accepting both MEDIA_BUS_FMT_RGB888_1X24 and
MEDIA_BUS_FMT_BGR888_1X24 as valid MBUS codes for RGB, alongside
MEDIA_BUS_FMT_UYVY8_1X16 for UYVY?
Dave
[1] https://pikvm.org/
[2] https://www.kernel.org/doc/html/latest/process/handling-regressions.html#quotes-from-linus-about-regression
> Fixes: d32d98642de6 ("[media] Driver for Toshiba TC358743 HDMI to CSI-2 bridge")
> Signed-off-by: Maxime Ripard <mripard@...nel.org>
> ---
> drivers/media/i2c/tc358743.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/media/i2c/tc358743.c b/drivers/media/i2c/tc358743.c
> index ca0b0b9bda1755313f066ba36ab218873b9ae438..a1c164a7716a10b0cb9ff38f88c0513b45f24771 100644
> --- a/drivers/media/i2c/tc358743.c
> +++ b/drivers/media/i2c/tc358743.c
> @@ -688,11 +688,11 @@ static void tc358743_set_csi_color_space(struct v4l2_subdev *sd)
> mutex_lock(&state->confctl_mutex);
> i2c_wr16_and_or(sd, CONFCTL, ~MASK_YCBCRFMT,
> MASK_YCBCRFMT_422_8_BIT);
> mutex_unlock(&state->confctl_mutex);
> break;
> - case MEDIA_BUS_FMT_RGB888_1X24:
> + case MEDIA_BUS_FMT_BGR888_1X24:
> v4l2_dbg(2, debug, sd, "%s: RGB 888 24-bit\n", __func__);
> i2c_wr8_and_or(sd, VOUT_SET2,
> ~(MASK_SEL422 | MASK_VOUT_422FIL_100) & 0xff,
> 0x00);
> i2c_wr8_and_or(sd, VI_REP, ~MASK_VOUT_COLOR_SEL & 0xff,
> @@ -1353,11 +1353,11 @@ static int tc358743_log_status(struct v4l2_subdev *sd)
> (i2c_rd16(sd, CSI_STATUS) & MASK_S_HLT) ?
> "yes" : "no");
> v4l2_info(sd, "Color space: %s\n",
> state->mbus_fmt_code == MEDIA_BUS_FMT_UYVY8_1X16 ?
> "YCbCr 422 16-bit" :
> - state->mbus_fmt_code == MEDIA_BUS_FMT_RGB888_1X24 ?
> + state->mbus_fmt_code == MEDIA_BUS_FMT_BGR888_1X24 ?
> "RGB 888 24-bit" : "Unsupported");
>
> v4l2_info(sd, "-----%s status-----\n", is_hdmi(sd) ? "HDMI" : "DVI-D");
> v4l2_info(sd, "HDCP encrypted content: %s\n",
> hdmi_sys_status & MASK_S_HDCP ? "yes" : "no");
> @@ -1691,11 +1691,11 @@ static int tc358743_enum_mbus_code(struct v4l2_subdev *sd,
> struct v4l2_subdev_state *sd_state,
> struct v4l2_subdev_mbus_code_enum *code)
> {
> switch (code->index) {
> case 0:
> - code->code = MEDIA_BUS_FMT_RGB888_1X24;
> + code->code = MEDIA_BUS_FMT_BGR888_1X24;
> break;
> case 1:
> code->code = MEDIA_BUS_FMT_UYVY8_1X16;
> break;
> default:
> @@ -1753,11 +1753,11 @@ static int tc358743_set_fmt(struct v4l2_subdev *sd,
>
> if (ret)
> return ret;
>
> switch (code) {
> - case MEDIA_BUS_FMT_RGB888_1X24:
> + case MEDIA_BUS_FMT_BGR888_1X24:
> case MEDIA_BUS_FMT_UYVY8_1X16:
> break;
> default:
> return -EINVAL;
> }
> @@ -2172,11 +2172,11 @@ static int tc358743_probe(struct i2c_client *client)
> sd->entity.function = MEDIA_ENT_F_VID_IF_BRIDGE;
> err = media_entity_pads_init(&sd->entity, 1, &state->pad);
> if (err < 0)
> goto err_hdl;
>
> - state->mbus_fmt_code = MEDIA_BUS_FMT_RGB888_1X24;
> + state->mbus_fmt_code = MEDIA_BUS_FMT_BGR888_1X24;
>
> sd->dev = &client->dev;
>
> mutex_init(&state->confctl_mutex);
>
>
> --
> 2.49.0
>
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