Message-ID: <CAA+D8ANb6A9eh=MQR9+7sZi5jet+7RSHt6TdZqPz5EK6pBs3mA@mail.gmail.com>
Date:   Thu, 16 Nov 2023 17:15:07 +0800
From:   Shengjiu Wang <shengjiu.wang@...il.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Shengjiu Wang <shengjiu.wang@....com>,
        Sakari Ailus <sakari.ailus@....fi>, m.szyprowski@...sung.com,
        mchehab@...nel.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiubo.Lee@...il.com,
        festevam@...il.com, nicoleotsuka@...il.com, lgirdwood@...il.com,
        broonie@...nel.org, perex@...ex.cz, tiwai@...e.com,
        alsa-devel@...a-project.org, linuxppc-dev@...ts.ozlabs.org,
        Tomasz Figa <tfiga@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v9 10/15] media: uapi: Add V4L2_CTRL_TYPE_FIXED_POINT

On Thu, Nov 16, 2023 at 4:01 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> Shengjiu,
>
> FYI: I started work on adding the fraction_bits field. I hope to have a
> patch for that early next week.
>
Thanks.  I will wait for your patch to be ready.

best regards
wang shengjiu

> Regards,
>
>         Hans
>
> On 16/11/2023 08:31, Tomasz Figa wrote:
> > On Wed, Nov 15, 2023 at 8:49 PM Laurent Pinchart
> > <laurent.pinchart@...asonboard.com> wrote:
> >>
> >> 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@....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
> >>>
> >
> > I like the idea and the name sounds fine to me too.
> >
> >>> 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.
> >>
> >
> > What usage do we envision for units? Could one give some examples? My
> > impression is that we already defined most of the controls with
> > explicit units.
> >
> >>>>>>>>> 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
>

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