[PATCH v9 10/15] media: uapi: Add V4L2_CTRL_TYPE_FIXED_POINT

[Laurent Pinchart]

On Mon, Nov 13, 2023 at 11:28:51AM +0000, Sakari Ailus wrote:
> Hi Hans,
> 
> On Mon, Nov 13, 2023 at 12:24:14PM +0100, Hans Verkuil wrote:
> > On 13/11/2023 12:07, Laurent Pinchart wrote:
> > > On Mon, Nov 13, 2023 at 11:56:49AM +0100, Hans Verkuil wrote:
> > >> On 13/11/2023 11:42, Laurent Pinchart wrote:
> > >>> On Mon, Nov 13, 2023 at 11:29:09AM +0100, Hans Verkuil wrote:
> > >>>> Hi Shengjiu,
> > >>>>
> > >>>> On 10/11/2023 06:48, Shengjiu Wang wrote:
> > >>>>> Fixed point controls are used by the user to configure
> > >>>>> a fixed point value in 64bits, which Q31.32 format.
> > >>>>>
> > >>>>> Signed-off-by: Shengjiu Wang <shengjiu.wang at nxp.com>
> > >>>>
> > >>>> This patch adds a new control type. This is something that also needs to be
> > >>>> tested by v4l2-compliance, and for that we need to add support for this to
> > >>>> one of the media test-drivers. The best place for that is the vivid driver,
> > >>>> since that has already a bunch of test controls for other control types.
> > >>>>
> > >>>> See e.g. VIVID_CID_INTEGER64 in vivid-ctrls.c.
> > >>>>
> > >>>> Can you add a patch adding a fixed point test control to vivid?
> > >>>
> > >>> I don't think V4L2_CTRL_TYPE_FIXED_POINT is a good idea. This seems to
> > >>> relate more to units than control types. We have lots of fixed-point
> > >>> values in controls already, using the 32-bit and 64-bit integer control
> > >>> types. They use various locations for the decimal point, depending on
> > >>> the control. If we want to make this more explicit to users, we should
> > >>> work on adding unit support to the V4L2 controls.
> > >>
> > >> "Fixed Point" is not a unit, it's a type. 'Db', 'Hz' etc. are units.
> > > 
> > > It's not a unit, but I think it's related to units. My point is that,
> > > without units support, I don't see why we need a formal definition of
> > > fixed-point types, and why this series couldn't just use
> > > VIVID_CID_INTEGER64. Drivers already interpret VIVID_CID_INTEGER64
> > > values as they see fit.
> > 
> > They do? That's new to me. A quick grep for V4L2_CTRL_TYPE_INTEGER64
> > (I assume you meant that rather than VIVID_CID_INTEGER64) shows that it

Yes, I meant V4L2_CTRL_TYPE_INTEGER64. Too hasty copy & paste :-)

> > is always interpreted as a 64 bit integer and nothing else. As it should.

The most common case for control handling in drivers is taking the
integer value and converting it to a register value, using
device-specific encoding of the register value. It can be a fixed-point
format or something else, depending on the device. My point is that
drivers routinely convert a "plain" integer to something else, and that
has never been considered as a cause of concern. I don't see why it
would be different in this series.

> > And while we do not have support for units (other than the documentation),
> > we do have type support in the form of V4L2_CTRL_TYPE_*.
> > 
> > >> A quick "git grep -i "fixed point" Documentation/userspace-api/media/'
> > >> only shows a single driver specific control (dw100.rst).
> > >>
> > >> I'm not aware of other controls in mainline that use fixed point.
> > > 
> > > The analog gain control for sensors for instance.
> > 
> > Not really. The documentation is super vague:
> > 
> > V4L2_CID_ANALOGUE_GAIN (integer)
> > 
> > 	Analogue gain is gain affecting all colour components in the pixel matrix. The
> > 	gain operation is performed in the analogue domain before A/D conversion.
> > 
> > And the integer is just a range. Internally it might map to some fixed
> > point value, but userspace won't see that, it's hidden in the driver AFAICT.

It's hidden so well that libcamera has a database of the sensor it
supports, with formulas to map a real gain value to the
V4L2_CID_ANALOGUE_GAIN control. The encoding of the integer value does
matter, and the kernel doesn't expose it. We may or may not consider
that as a shortcoming of the V4L2 control API, but in any case it's the
situation we have today.

> I wonder if Laurent meant digital gain.

No, I meant analog. It applies to digital gain too though.

> Those are often Q numbers. The practice there has been that the default
> value yields gain of 1.
> 
> There are probably many other examples in controls where something being
> controlled isn't actually an integer while integer controls are still being
> used for the purpose.

A good summary of my opinion :-)

> Instead of this patch, I'd prefer to have a way to express the meaning of
> the control value, be it a Q number or something else, and do that
> independently of the type of the control.

Agreed.

> > In the case of this particular series the control type is really a fixed point
> > value with a documented unit (Hz). It really is not something you want to
> > use type INTEGER64 for.
> > 
> > >> Note that V4L2_CTRL_TYPE_FIXED_POINT is a Q31.32 format. By setting
> > >> min/max/step you can easily map that to just about any QN.M format where
> > >> N <= 31 and M <= 32.
> > >>
> > >> In the case of dw100 it is a bit different in that it is quite specialized
> > >> and it had to fit in 16 bits.

-- 
Regards,

Laurent Pinchart