Hi Hans, 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. > >> 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
