Shengjiu, FYI: I started work on adding the fraction_bits field. I hope to have a patch for that early next week. Regards, Hans On 16/11/2023 08:31, Tomasz Figa wrote: > On Wed, Nov 15, 2023 at 8:49 PM Laurent Pinchart > <laurent.pinchart@xxxxxxxxxxxxxxxx> 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@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 >>> > > 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
