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Date:   Wed, 15 Nov 2023 13:49:31 +0200
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Hans Verkuil <hverkuil@xs4all.nl>
Cc:     Sakari Ailus <sakari.ailus@iki.fi>,
        Shengjiu Wang <shengjiu.wang@nxp.com>, tfiga@chromium.org,
        m.szyprowski@samsung.com, mchehab@kernel.org,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        shengjiu.wang@gmail.com, Xiubo.Lee@gmail.com, festevam@gmail.com,
        nicoleotsuka@gmail.com, lgirdwood@gmail.com, broonie@kernel.org,
        perex@perex.cz, tiwai@suse.com, alsa-devel@alsa-project.org,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v9 10/15] media: uapi: Add V4L2_CTRL_TYPE_FIXED_POINT
Message-ID: <20231115114931.GE13826@pendragon.ideasonboard.com>
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Hi Hans,

On Wed, Nov 15, 2023 at 12:19:31PM +0100, Hans Verkuil wrote:
> On 11/15/23 11:55, Laurent Pinchart wrote:
> > On Wed, Nov 15, 2023 at 09:09:42AM +0100, Hans Verkuil wrote:
> >> On 13/11/2023 13:44, Laurent Pinchart wrote:
> >>> On Mon, Nov 13, 2023 at 01:05:12PM +0100, Hans Verkuil wrote:
> >>>> On 13/11/2023 12:43, Laurent Pinchart wrote:
> >>>>> On Mon, Nov 13, 2023 at 11:28:51AM +0000, Sakari Ailus wrote:
> >>>>>> 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:
> >>>>>>>>>>> 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@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 :-)
> >>>>
> >>>> And that works fine as long as userspace doesn't need to know what the value
> >>>> actually means.
> >>>>
> >>>> That's not the case here. The control is really a fractional Hz value:
> >>>>
> >>>> +``V4L2_CID_M2M_AUDIO_SOURCE_RATE_OFFSET (fixed point)``
> >>>> +    Sets the offset from the audio source sample rate, unit is Hz.
> >>>> +    The offset compensates for any clock drift. The actual source audio sample
> >>>> +    rate is the ideal source audio sample rate from
> >>>> +    ``V4L2_CID_M2M_AUDIO_SOURCE_RATE`` plus this fixed point offset.
> >>>
> >>> I don't see why this would require a new type, you can use
> >>> V4L2_CTRL_TYPE_INTEGER64, and document the control as containing
> >>> fixed-point values in Q31.32 format.
> >>
> >> Why would you want to do this? I can store a double in a long long int,
> >> and just document that the variable is really a double, but why would you?
> > 
> > I'm happy we have no floating point control types ;-)
> > 
> >> The cost of adding a FIXED_POINT type is minimal, and having this type
> >> makes it easy to work with fixed point controls (think about proper reporting
> >> and setting of the value in v4l2-ctl and user applications in general that
> >> deal with controls).
> > 
> > The next thing you know is that someone will want a FIXED_POINT_Q15_16
> > type as 64-bit would be too large to store in a large array. And then
> > Q7.8. And Q3.12. And a bunch of other type. I really don't see what
> > added value they bring compared to using the 32-bit and 64-bit integer
> > types we already have. Every new type that is added adds complexity to
> > userspace that will need to deal with the type.
> > 
> >> If this would add a thousand lines of complex code, then this would be a
> >> consideration, but this is just a few lines.
> >>
> >> Just to give an example, if you use 'v4l2-ctl -l' to list a int64 control
> >> and it reports the value 13958643712, would you be able to see that that is
> >> really 3.25 in fixed point format? With the right type it would be printed
> >> like that. Much easier to work work.
> > 
> > The same is true for analog gains, where x1.23 or +12dB is nicer to read
> > than raw values. If we care about printing values in command line tools
> > (which is nice to have, but certainly not the majority of use cases),
> > then I would recommand working on units support for V4L2 controls, to
> > convey how values are encoded, and in what unit they are expressed.
> 
> So you prefer to have a way to specify the N value in QM.N as part
> of the control information?
> 
> E.g. add a '__u8 fraction_bits' field to structs v4l2_query_ext_ctrl
> and v4l2_queryctrl. If 0, then it is an integer, otherwise it is the N
> in QM.N.
> 
> I can go along with that. This would be valid for INTEGER, INTEGER64,
> U8, U16 and U32 controls (the last three are only used in control arrays).

I think that would be nicer. Not only is it more flexible, but it also
allows applications to ignore that information, and still operate on
integer controls without any modification.

> A better name for 'fraction_bits' is welcome, I took it from the wikipedia
> article: https://en.wikipedia.org/wiki/Fixed-point_arithmetic
> 
> Reporting unit names is certainly possible, but should perhaps be done
> with a separate ioctl? E.g. VIDIOC_QUERY_CTRL_UNIT. It is not typically
> needed for applications, unless they need to report values. In theory
> it can also be reported through VIDIOC_QUERY_EXT_CTRL by using, say,
> 4 of the reserved fields for a 'char unit[16];' field. But I feel a
> bit uncomfortable taking reserved fields for something that is rarely
> needed.

I would make the unit an enumerated integer value. If it's a string, it
gets more difficult to operate on. Having to standardize a unit means
that the unit will get reviewed.

> >>>>>> 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.
> >>>>
> >>>> Huh? How is that different from the type of the control? You have integers
> >>>> (one type) and fixed point (another type).
> >>>>
> >>>> Or do you want a more general V4L2_CTRL_TYPE_ that specifies the N.M values
> >>>> explicitly?
> >>>>
> >>>> I think the main reason why we use integer controls for gain is that we
> >>>> never had a fixed point control type and you could get away with that in
> >>>> user space for that particular use-case.
> >>>>
> >>>> Based on the V4L2_CID_NOTIFY_GAINS documentation the gain value can typically
> >>>> be calculated as (value / default_value),
> >>>
> >>> Typically, but not always. Some sensor have an exponential gain model,
> >>> and some have weird gain representation, such as 1/x. That's getting out
> >>> of scope though.
> >>>
> >>>> but that won't work for a rate offset
> >>>> control as above, or for e.g. CSC matrices for color converters.
> >>>>
> >>>>> 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