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@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). 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. >
