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Message-ID: <a67491c0-4fdf-4472-852c-e75f5e1d67af@xs4all.nl>
Date:   Wed, 15 Nov 2023 12:19:31 +0100
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Subject: Re: [PATCH v9 10/15] media: uapi: Add V4L2_CTRL_TYPE_FIXED_POINT
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To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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
References: <4cd6b593-2376-4cbc-a7c8-d3eb36a2f7a0@xs4all.nl>
 <20231113104238.GA13981@pendragon.ideasonboard.com>
 <6a3e7eb9-505c-4cfb-8a86-a8947a2e44d5@xs4all.nl>
 <20231113110754.GB24338@pendragon.ideasonboard.com>
 <3e898664-cbfc-4892-9765-37b66891643b@xs4all.nl>
 <ZVIIc-fi32ZxIi-p@valkosipuli.retiisi.eu>
 <20231113114357.GD24338@pendragon.ideasonboard.com>
 <da6efe14-c00d-4bf4-bf61-dd4ed39c5c60@xs4all.nl>
 <20231113124412.GA18974@pendragon.ideasonboard.com>
 <b35601f7-8bb2-4317-a8f7-6fbf81572943@xs4all.nl>
 <20231115105518.GD13826@pendragon.ideasonboard.com>
From:   Hans Verkuil <hverkuil@xs4all.nl>
In-Reply-To: <20231115105518.GD13826@pendragon.ideasonboard.com>
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On 11/15/23 11:55, Laurent Pinchart wrote:
> Hi Hans,
> 
> 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).

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.

Regards,

	Hans

> 
>>>>>> 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.
>