Messages in this thread | | | Date | Tue, 31 Mar 2020 15:14:52 +0200 | From | Clemens Gruber <> | Subject | Re: (EXT) Re: [PATCH 2/4] pwm: pca9685: remove ALL_LED PWM channel |
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On Tue, Mar 31, 2020 at 02:09:37PM +0200, Matthias Schiffer wrote: > On Mon, 2020-03-30 at 18:07 +0200, Clemens Gruber wrote: > > On Mon, Mar 30, 2020 at 05:40:36PM +0200, Thierry Reding wrote: > > > On Mon, Mar 30, 2020 at 03:34:50PM +0200, Clemens Gruber wrote: > > > > Hi, > > > > > > > > On Mon, Mar 30, 2020 at 03:07:57PM +0200, Thierry Reding wrote: > > > > > On Wed, Feb 26, 2020 at 02:52:27PM +0100, Matthias Schiffer > > > > > wrote: > > > > > > The interaction of the ALL_LED PWM channel with the other > > > > > > channels was > > > > > > not well-defined. As the ALL_LED feature does not seem very > > > > > > useful and > > > > > > it was making the code significantly more complex, simply > > > > > > remove it. > > > > > > > > > > > > Signed-off-by: Matthias Schiffer < > > > > > > matthias.schiffer@ew.tq-group.com> > > > > > > --- > > > > > > drivers/pwm/pwm-pca9685.c | 115 ++++++-------------------- > > > > > > ------------ > > > > > > 1 file changed, 17 insertions(+), 98 deletions(-) > > > > > > > > > > Applied, thanks. > > > > > > > > > > Thierry > > > > > > > > I was not reading the mailing list in the last weeks, so I only > > > > saw the > > > > patch today. > > > > > > > > We are using the ALL_LED channel in production to reduce the > > > > delay when > > > > all 16 PWM outputs need to be set to the same duty cycle. > > > > > > > > I am not sure it is a good idea to remove this feature. > > > > > > Can you specify what platform this is and where the code is that > > > does > > > this. I can't really find any device tree users of this and I don't > > > know > > > if there's a good way to find out what other users there are, but > > > this > > > isn't the first time this driver has created confusion, so please > > > help > > > collect some more information about it's use so we can avoid this > > > in the > > > future. > > > > The platform is ARM, it's a custom board with an NXP i.MX6. The > > device > > tree is not upstreamed. As there are multiple companies involved > > in changes to this driver, I assume that it is in use, even though > > there > > are no in-tree users. > > Also: As you can set the ALL channel from userspace, it will be very > > difficult to find out how often the ALL feature is being used > > somewhere. > > > > > > > > I'll back out this particular patch since you're using it. Can you > > > give > > > the other three patches a try to see if they work for you? > > > > Thanks! I saw your other mail. Patch 1 looks good to me. I will look > > at > > the new version of patches 3 and 4 and test them when they appear on > > the > > list. > > > > Clemens > > Thanks for the feedback, I'll have to respin my cleanup patches without > removing this feature. > > I wonder if we can come up with a sane semantics of how ALL_LED is > supposed to interact with the individual channels? Optimally, changes > made via ALL_LED should be reflected in the state of the other channels > including their sysfs files, but I'm not sure if current APIs can > support this cleanly. It might make sense to make requesting/exporting > individual channels and ALL_LED mutually exclusive, so the state of a > requested PWM can't change when it's supposed to be under exclusive > control of one user. Of course, such a change can break existing users > as well...
I agree that it would be a good idea to make this exclusive. This change would at least not break our application, because we unexport the ALL_LED channel before requesting an individual channel. Not sure about other users, but using both individual and ALL channels at the same time is probably not a reasonable/sane usecase..
> And what about state propagation in the other direction - how should > the ALL_LED state reflect changes made to the other channels' settings? > On the hardware side, the ALL_LED registers are write-only, as there > aren't any sane values that could be returned.
According to the datashet (7.3.4) the individual registers are filled if the ALL_LED channel is used. However, in .disable, the OFF registers are reset to 0. (And the ON registers are not used, except for the FULL_ON bit) So there should not be any side effects, as long as the access to the ALL_LED channel is made exclusive and the user has to free it before he can request individual channels.
Another quirk is the same prescaler/period for all channels. But I am not sure what we can do about that. Applications might already depend on the fact that the last set period wins.
Clemens
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