On Mon, Apr 29, 2019 at 01:03:54PM +0200, Thierry Reding wrote: > On Mon, Apr 08, 2019 at 04:39:14PM +0200, Uwe Kleine-König wrote: > > On Mon, Apr 01, 2019 at 03:45:47PM -0700, Doug Anderson wrote: > > > Hi, > > > > > > On Sat, Mar 30, 2019 at 2:17 AM Heiko Stuebner <heiko@xxxxxxxxx> wrote: > > > > > > > > Hi, > > > > > > > > [adding two chromeos people, because veyron and gru are quite > > > > heavy users of the rockchip pwm for both backlight and regulators] > > > > > > > > Doug, Brian: patchwork patch is here: > > > > https://patchwork.kernel.org/patch/10851001/ > > > > > > > > Am Dienstag, 12. März 2019, 22:46:03 CET schrieb Uwe Kleine-König: > > > > > The pwm-rockchip driver is one of only two PWM drivers which updates the > > > > > state for the caller of pwm_apply_state(). This might have surprising > > > > > results if the caller reuses the values expecting them to still > > > > > represent the same state. > > > > > > It may or may not be surprising, but it is well documented. Specifically: > > > > > > * pwm_apply_state() - atomically apply a new state to a PWM device > > > * @pwm: PWM device > > > * @state: new state to apply. This can be adjusted by the PWM driver > > > * if the requested config is not achievable, for example, > > > * ->duty_cycle and ->period might be approximated. > > > > > > I don't think your series updates that documentation, right? > > > > > > > > > > > Also note that this feedback was incomplete as > > > > > the matching struct pwm_device::state wasn't updated and so > > > > > pwm_get_state still returned the originally requested state. > > > > > > The framework handles that. Take a look at pwm_apply_state()? It does: > > > > > > --- > > > > > > err = pwm->chip->ops->apply(pwm->chip, pwm, state); > > > if (err) > > > return err; > > > > > > pwm->state = *state; > > > > > > > > --- > > > > > > So I think it wasn't incomplete unless I misunderstood? > > > > You're right, I missed that the updated state was saved. > > > > > > > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> > > > > > > > > I've tested this on both veyron and gru with backlight and pwm regulator > > > > and at least both still come up, so > > > > Tested-by: Heiko Stuebner <heiko@xxxxxxxxx> > > > > > > > > But hopefully Doug or Brian could also provide another test-point. > > > > > > I'd definitely be concerned by this change. Specifically for the PWM > > > regulator little details about exactly what duty cycle / period you > > > got could be pretty important. > > > > > > I guess the problem here is that pwm_get_state() doesn't actually call > > > into the PWM drivers, it just returns the last state that was applied. > > > How does one get the state? I guess you could change get_state() to > > > actually call into the PWM driver's get_state if it exists? ...but > > > your patch set doesn't change that behavior... > > > > My intention here is more to make all drivers behave the same way and > > because only two drivers updated the pwm_state this was the variant I > > removed. > > > > When you say that the caller might actually care about the exact > > parameters I fully agree. In this case however the consumer should be > > able to know the result before actually applying it. So if you do > > > > pwm_apply_state(pwm, { .period = 17, .duty_cycle = 12, ...}) > > > > and this results in .period = 100 and .duty_cycle = 0 then probably the > > bad things you want to know about already happend. So my idea is a new > > function pwm_round_state() that does the adaptions to pwm_state without > > applying it to the hardware. After that pwm_apply_state could do the > > following: > > > > rstate = pwm_round_state(pwm, state) > > pwm.apply(pwm, state) > > gstate = pwm_get_state(pwm) > > > > if rstate != gstate: > > warn about problems > > I'm not sure this is really useful. I would expect that in most cases > where it is necessary to have an exact match between the requested state > and the actual state is important, you may not even get to warning about > problems because the system may shut down (e.g. the regulator might not > be outputting enough power to keep the system stable). > > I think it'd be far more useful to give consumers a way to request that > the state be applied strictly. I'm not sure how realistic that is, > though. Most PWMs have some sort of restrictions, and in most cases this > might still be okay. Perhaps some sort of permissible relative deviation > factor could be added to give more flexibility? I think in practise this isn't going to work. Consider the case that Brian cares about: "we do need to be as precise as possible with the duty:period ratio". So if we want 1/5 duty we might request: .duty_cycle = 100, .period = 500 an are using pwm_set_state_exact(). Now consider this fails. What is the next value you should try? It's hard to say without knowing why it failed. If however you could do mystate.duty_cycle = 100 mystate.period = 500 pwm_round_state(pwm, &mystate); and after that we have: mystate.duty_cycle = 99; mystate.period = 498; (because pwm_round_state is supposed to round down[1] and the hardware can implement multiples of 3 only). Then it is easier to determine the next state to try. Best regards Uwe [1] this isn't fixed yet, but I think it's sensible. -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | _______________________________________________ Linux-rockchip mailing list Linux-rockchip@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/linux-rockchip
