linux-kernel - Re: [PATCH v7 3/3] media: i2c: Introduce a driver for the Techwell TW9900 decoder

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Date:   Mon, 6 Nov 2023 16:25:18 +0100
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Mehdi Djait <mehdi.djait@...tlin.com>
Cc:     mchehab@...nel.org, hverkuil-cisco@...all.nl,
        krzysztof.kozlowski+dt@...aro.org, robh+dt@...nel.org,
        conor+dt@...nel.org, laurent.pinchart@...asonboard.com,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
        alexandre.belloni@...tlin.com, maxime.chevallier@...tlin.com
Subject: Re: [PATCH v7 3/3] media: i2c: Introduce a driver for the Techwell
 TW9900 decoder

Hi Mehdi,

On Mon 06 Nov 23, 15:58, Mehdi Djait wrote:
> Hi Paul,
> 
> thank you for the review!
> 
> On Thu, Nov 02, 2023 at 11:03:42AM +0100, Paul Kocialkowski wrote:
> > > +static int tw9900_write_reg(struct i2c_client *client, u8 reg, u8 val)
> > > +{
> > > +	int ret;
> > > +
> > > +	ret = i2c_smbus_write_byte_data(client, reg, val);
> > 
> > Is this an SMBUS device in particular? Or is there any reason to use the SMBUS
> > API instead of the general I2C API?
> > 
> 
> I think I will keep using the SMBUS API here. The reason is in the
> kernel documentation:
> 
> --------------------------------------------------------------------------------
> If you write a driver for some I2C device, please try to use the SMBus commands 
> if at all possible (if the device uses only that subset of the I2C protocol). 
> This makes it possible to use the device driver on both SMBus adapters and I2C 
> adapters (the SMBus command set is automatically translated to I2C on I2C 
> adapters, but plain I2C commands can not be handled at all on most pure SMBus 
> adapters).
> --------------------------------------------------------------------------------
> 
> And the vast majority of the drivers under /media/i2c are using the
> SMBUS API.

That is a good reason, so let's keep it that way then.

> > > +static void tw9900_fill_fmt(const struct tw9900_mode *mode,
> > > +			    struct v4l2_mbus_framefmt *fmt)
> > > +{
> > > +	fmt->code = MEDIA_BUS_FMT_UYVY8_2X8;
> > > +	fmt->width = mode->width;
> > > +	fmt->height = mode->height;
> > > +	fmt->field = V4L2_FIELD_NONE;
> > > +	fmt->quantization = V4L2_QUANTIZATION_DEFAULT;
> > > +	fmt->colorspace = V4L2_COLORSPACE_SMPTE170M;
> > > +	fmt->xfer_func = V4L2_MAP_XFER_FUNC_DEFAULT(V4L2_COLORSPACE_SMPTE170M);
> > > +	fmt->ycbcr_enc = V4L2_MAP_YCBCR_ENC_DEFAULT(V4L2_COLORSPACE_SMPTE170M);
> > > +}
> > > +
> > > +static int tw9900_cfg_fmt(struct v4l2_subdev *sd,
> > 
> > You might have to differentiate between set_fmt/get_fmt to return -EBUSY
> > if streaming is on in set_fmt. However I understand it will just copy the
> > current mode in both cases, but this might still be required to follow v4l2
> > semantics (please double-check).
> > 
> 
> This should be done in the driver calling the pad subdev_call set_fmt,
> right ?

Well the two things are distinct, even though it's not obvious to think about
a case where you wouldn't have a video device to grab the frames.

For instance you can see this being done here:
https://elixir.bootlin.com/linux/latest/source/drivers/media/i2c/ov5648.c#L2259

I'm just not sure about what the V4L2 subdev API mandates. It would be useful
to find some piece of documentation that clarifies the requirement.

Cheers,

Paul

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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