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@xxxxxxx> > >>>>>>>>>>> > >>>>>>>>>>> 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
