On Mon, Nov 13, 2023 at 11:28:51AM +0000, Sakari Ailus wrote: > Hi Hans, > > 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: > > >>>> Hi Shengjiu, > > >>>> > > >>>> 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 :-) > 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. 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
