linux-kernel - Re: [PATCH] media: i2c: imx219: Implement V4L2_CID_LINK

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Date:   Tue, 26 Jan 2021 13:01:08 +0000
From:   Dave Stevenson <dave.stevenson@...pberrypi.com>
To:     Andrey Konovalov <andrey.konovalov@...aro.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Peter Griffin <peter.griffin@...aro.org>
Subject: Re: [PATCH] media: i2c: imx219: Implement V4L2_CID_LINK_FREQ control

Hi Andrey

On Tue, 26 Jan 2021 at 07:50, Andrey Konovalov
<andrey.konovalov@...aro.org> wrote:
>
> This control is needed for imx219 driver, as the link frequency
> is independent from the pixel rate in this case, and can't be
> calculated from the pixel rate.
>
> Signed-off-by: Andrey Konovalov <andrey.konovalov@...aro.org>
> ---
>  drivers/media/i2c/imx219.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/media/i2c/imx219.c b/drivers/media/i2c/imx219.c
> index 92a8d52776b8..6e3382b85a90 100644
> --- a/drivers/media/i2c/imx219.c
> +++ b/drivers/media/i2c/imx219.c
> @@ -390,6 +390,10 @@ static const struct imx219_reg raw10_framefmt_regs[] = {
>         {0x0309, 0x0a},
>  };
>
> +static const s64 imx219_link_freq_menu[] = {
> +       IMX219_DEFAULT_LINK_FREQ,

Link frequency is one of the parameters that is largely irrelevant on
the Pi, so I've partially ignored it.

Is the link frequency really the same for all modes? Even 8 bit vs 10
bit readout?

The pixel rate is constant at 182.4Mpix/s for all modes.
Switching to 8 bit changes register 0x0309 (op_pix_clk_div) from 10 to 8.
Figure 43 "Clock System Block Diagram" in the datasheet I have says
this reduces the divider to the FIFO between the pipeline and MIPI. As
we haven't changed PLL2 or Pre-div2 I'd expect the link frequency to
stay the same, but that leaves me confused over that FIFO clock as
it'll go UP in frequency. I can't quite see how that works, but it
clearly does.

Both 8 and 10 bit modes do read out at the same frame / pixel rate,
therefore that bit is correct, but that leaves me puzzling over link
frequency. I have no information on how big that FIFO is, or how it's
clocked on input and output.

Simplest option is that as I need to go into the office in the next
day or so I'll pop into the lab and measure it in each mode.

Otherwise I have no issues with the implementation of the patch.

  Dave

> +};
> +
>  static const char * const imx219_test_pattern_menu[] = {
>         "Disabled",
>         "Color Bars",
> @@ -547,6 +551,7 @@ struct imx219 {
>         struct v4l2_ctrl_handler ctrl_handler;
>         /* V4L2 Controls */
>         struct v4l2_ctrl *pixel_rate;
> +       struct v4l2_ctrl *link_freq;
>         struct v4l2_ctrl *exposure;
>         struct v4l2_ctrl *vflip;
>         struct v4l2_ctrl *hflip;
> @@ -1269,7 +1274,7 @@ static int imx219_init_controls(struct imx219 *imx219)
>         int i, ret;
>
>         ctrl_hdlr = &imx219->ctrl_handler;
> -       ret = v4l2_ctrl_handler_init(ctrl_hdlr, 11);
> +       ret = v4l2_ctrl_handler_init(ctrl_hdlr, 12);
>         if (ret)
>                 return ret;
>
> @@ -1283,6 +1288,14 @@ static int imx219_init_controls(struct imx219 *imx219)
>                                                IMX219_PIXEL_RATE, 1,
>                                                IMX219_PIXEL_RATE);
>
> +       imx219->link_freq =
> +               v4l2_ctrl_new_int_menu(ctrl_hdlr, &imx219_ctrl_ops,
> +                                      V4L2_CID_LINK_FREQ,
> +                                      ARRAY_SIZE(imx219_link_freq_menu) - 1, 0,
> +                                      imx219_link_freq_menu);
> +       if (imx219->link_freq)
> +               imx219->link_freq->flags |= V4L2_CTRL_FLAG_READ_ONLY;
> +
>         /* Initial vblank/hblank/exposure parameters based on current mode */
>         imx219->vblank = v4l2_ctrl_new_std(ctrl_hdlr, &imx219_ctrl_ops,
>                                            V4L2_CID_VBLANK, IMX219_VBLANK_MIN,
> --
> 2.17.1
>