linux-kernel - Re: [PATCH v2 1/3] media: mt9m111: add V4L2_CID_LINK

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Date:   Tue, 20 Sep 2022 11:43:37 +0200
From:   Jacopo Mondi <jacopo@...ndi.org>
To:     Marco Felsch <m.felsch@...gutronix.de>
Cc:     mchehab@...nel.org, sakari.ailus@...ux.intel.com,
        laurent.pinchart+renesas@...asonboard.com, akinobu.mita@...il.com,
        jacopo+renesas@...ndi.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] media: mt9m111: add V4L2_CID_LINK_FREQ support

Hi Marco

On Fri, Sep 16, 2022 at 03:57:11PM +0200, Marco Felsch wrote:
> Add support to report the link frequency.
>
> Signed-off-by: Marco Felsch <m.felsch@...gutronix.de>
> ---
> The v1 of this small series can be found here:
> https://lore.kernel.org/all/20220818144712.997477-1-m.felsch@pengutronix.de/
>
> Thanks a lot to Jacopo for the review feedback on my v1.
>
> Changelog:
>
> v2:
> - use V4L2_CID_LINK_FREQ instead of V4L2_CID_PIXEL_RATE
> ---
>  drivers/media/i2c/mt9m111.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/media/i2c/mt9m111.c b/drivers/media/i2c/mt9m111.c
> index afc86efa9e3e..52be1c310455 100644
> --- a/drivers/media/i2c/mt9m111.c
> +++ b/drivers/media/i2c/mt9m111.c
> @@ -1249,6 +1249,8 @@ static int mt9m111_probe(struct i2c_client *client)
>  {
>  	struct mt9m111 *mt9m111;
>  	struct i2c_adapter *adapter = client->adapter;
> +	static s64 extclk_rate;

Why static ?
Also clk_get_rate() returns an unsigned long


> +	struct v4l2_ctrl *ctrl;
>  	int ret;
>
>  	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_WORD_DATA)) {
> @@ -1271,6 +1273,13 @@ static int mt9m111_probe(struct i2c_client *client)
>  	if (IS_ERR(mt9m111->clk))
>  		return PTR_ERR(mt9m111->clk);
>
> +	ret = clk_prepare_enable(mt9m111->clk);
> +	if (ret < 0)
> +		return ret;
> +
> +	extclk_rate = clk_get_rate(mt9m111->clk);
> +	clk_disable_unprepare(mt9m111->clk);
> +
>  	mt9m111->regulator = devm_regulator_get(&client->dev, "vdd");
>  	if (IS_ERR(mt9m111->regulator)) {
>  		dev_err(&client->dev, "regulator not found: %ld\n",
> @@ -1285,7 +1294,7 @@ static int mt9m111_probe(struct i2c_client *client)
>  	mt9m111->subdev.flags |= V4L2_SUBDEV_FL_HAS_DEVNODE |
>  				 V4L2_SUBDEV_FL_HAS_EVENTS;
>
> -	v4l2_ctrl_handler_init(&mt9m111->hdl, 7);
> +	v4l2_ctrl_handler_init(&mt9m111->hdl, 8);
>  	v4l2_ctrl_new_std(&mt9m111->hdl, &mt9m111_ctrl_ops,
>  			V4L2_CID_VFLIP, 0, 1, 1, 0);
>  	v4l2_ctrl_new_std(&mt9m111->hdl, &mt9m111_ctrl_ops,
> @@ -1309,6 +1318,16 @@ static int mt9m111_probe(struct i2c_client *client)
>  				BIT(V4L2_COLORFX_NEGATIVE) |
>  				BIT(V4L2_COLORFX_SOLARIZATION)),
>  			V4L2_COLORFX_NONE);

Empty line maybe ?

> +	/*
> +	 * The extclk rate equals the link freq. if reg default values are used,
> +	 * which is the case. This must be adapted as soon as we don't use the
> +	 * default values anymore.
> +	 */
> +	ctrl = v4l2_ctrl_new_int_menu(&mt9m111->hdl, &mt9m111_ctrl_ops,
> +				      V4L2_CID_LINK_FREQ, 0, 0, &extclk_rate);
> +	if (ctrl)
> +		ctrl->flags |= V4L2_CTRL_FLAG_READ_ONLY;
> +

I'm sorry I have not replied to your previous email about using
LINK_FREQ for parallel busses.. I see it mentioned in ext-ctrls-image-process.rst
as you said:

``V4L2_CID_LINK_FREQ (integer menu)``
    The frequency of the data bus (e.g. parallel or CSI-2).

I still have a bit of troubles seeing it apply nicely on a parallel
bus. Isn't PIXEL_RATE more appropriate ? You said you need to know the
overall bus bandwidth in bytes , and pixel_rate * bpp / 8 is equally
valid and easy as link_freq / num_lanes, which requires the receiver
to fetch the remote subdev media bus configuration instead of relying
on the input format. Also LINK_FREQ is a menu control, something nasty
already for CSI-2 busses, which requires to pre-calculate the link
freqs based on the input mclk. It is also meant to be changed by
userspace, while PIXEL_RATE is RO by default.

Sakari, Laurent, what's your take here ?



>  	mt9m111->subdev.ctrl_handler = &mt9m111->hdl;
>  	if (mt9m111->hdl.error) {
>  		ret = mt9m111->hdl.error;
> --
> 2.30.2
>