On Mon, 14 Dec 2020 16:30:22 +0100 Peter Rosin <peda@xxxxxxxxxx> wrote: > On 2020-12-14 16:07, Jonathan Cameron wrote: > > On Mon, 14 Dec 2020 09:34:40 +0100 > > Peter Rosin <peda@xxxxxxxxxx> wrote: > > > >> On 2020-12-13 13:16, Jonathan Cameron wrote: > >>> On Sun, 13 Dec 2020 00:22:17 +0100 > >>> Peter Rosin <peda@xxxxxxxxxx> wrote: > >>> > >>>> On 2020-12-12 13:26, Linus Walleij wrote: > >>>>> On Mon, Nov 2, 2020 at 12:22 AM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote: > >>>>> > >>>>>> It happens that an ADC will only provide raw or processed > >>>>>> voltage conversion channels. (adc/ab8500-gpadc.c). > >>>>>> On the Samsung GT-I9070 this is used for a light sensor > >>>>>> and current sense amplifier so we need to think of something. > >>>>>> > >>>>>> The idea is to allow processed channels and scale them > >>>>>> with 1/1 and then the rescaler can modify the result > >>>>>> on top. > >>>>>> > >>>>>> Cc: Peter Rosin <peda@xxxxxxxxxx> > >>>>>> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx> > >>>>> > >>>>> Did we reach any conclusion on this? I really need to use > >>>>> the rescaler on an ADC that only handles processed channels... > >>>>> > >>>>> I'm sorry that I can't make this ADC disappear :D > >>>> > >>>> Hi! > >>>> > >>>> My conclusion was that the patch is buggy since it presents inconsistent > >>>> information. That needs to be fixed one way or the other. If the offending > >>>> information cannot be filtered out for some reason, I don't know what to > >>>> do. Details in my previous comment [1]. BTW, I still do not know the answer > >>>> to the .read_avail question at the end of that message, and I don't have > >>>> time to dig into it. Sorry. > >>> > >>> Unless I'm missing something, I think it presents no information unless > >>> we strangely have a driver providing read_avail for _RAW but only > >>> _PROCESSED channels which is a bug. I'm not that bothered about > >>> missing information in this particular, somewhat obscure, corner case. > >>> > >>> So I think we should take the patch as it stands. It's missed the > >>> merge window now anyway unfortunately. So Peter, I would suggest we > >>> take this and perhaps revisit to tidy up loose corners when we all have > >>> more time. > >> > >> My concern was a driver with a raw channel, including read_avail, providing > >> raw sample values but that no easy conversion existed to get from that to > >> the processed values. One option for the driver in that case would be to > >> provide these raw values, but then have no scaling info. > > > > Generally I resist this a lot. The reason is that it is impossible to write > > generic userspace software against it. The one time we did let this happen > > was with some of the heart rate sensors (pulse oximeters) where the algorithm > > to derive the eventual value is both complex - based on published literature, > > and proprietary (what was actually readily usable). What the measurement being > > provided to userspace was is well documented, but not how on earth you get from > > that to something useable for what the sensor is designed to measure. > > > >> I.e. the way I see > >> it, it is perfectly reasonable for a driver to provide raw with read_avail, > >> no scaling but also processed values. > > > > Why? What use would the raw values actually be? There are a couple of historical > > drivers where they evolved to this state, but it is not one we would normally accept. > > We go to a lot of effort to try and avoid this. > > Drivers that have eveloved over time is exactly one such reason. E.g. a driver > starts out by not caring about wrong measurements at one end of the spectrum > because it is "linear enough" for the first use, someone comes along and fixes > that. But by that time it's impossible to completely remove the raw channel > because that would be a regression for some reason. And there you are. A > driver with raw plus read_avail, no scaling but a processed channel. Or > something like that... Yup, that's pretty much what tends to happen. I've gotten a lot stricter on checking datasheets to try and stop this happening, but still possible more will slip through (particularly as can't always get the datasheet) > > >> And that gets transformed by the > >> rescaler into the processed values being presented as raw, with rescaling > >> added on top, but with the read_avail info for this new raw channel being > >> completely wrong. > >> > >> For the intended driver (ab8500-gpadc) this is not the case (it has no > >> read_avail for its raw channel). But it does have a raw channel, so adding > >> read_avail seems easy and I can easily see other drivers already doing it. > >> Haven't checked that though... > > > > Drat. I'd failed to register this is one of those corner cases. > > I'm not sure, I just browsed the code. Maybe I misread it? It's doing both - you were right. I think there are only a small number of drivers that have that history. Looks superficially like it's easy enough to catch this corner case and block it - so lets do that. Jonathan > > Cheers, > Peter > > >> But if you say that this never happens, fine. Otherwise, since it's too > >> late for the merge window anyway, the patch might as well be updated such > >> that the rescaler blocks the read_avail channel in this situation, if it > >> exists. > > > > That's fair enough. A sanity check and then suitable warning message to explain > > why it is blocked makes sense. >
