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? The documentation is arguably ambiguous here, but I don't think this was meant as you intepret here. I think the original intent was to give the drivers some leeway in how they apply a state. So a driver could for example adjust duty_cycle or period if it doesn't support exactly the combination requested. However, I don't think it was meant as a suggestion that it would report that back to the caller. This obviously implies that ->apply() is deterministic, so given a state it would program the same register values, irrespective of when, or how many times that state is applied. > > > 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? > > > > > 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... > > For instance, look at pwm_regulator_set_voltage(). The first line > there is pwm_init_state(). That calls into pwm_get_state() which > just grabs the cached state. If the last call to pwm_apply_state() > didn't update that then it seems like it'd be bad. The whole point of this atomic API is that the cached state would always match the hardware state. Given what I said above that doesn't necessarily mean that the cached state exactly matches the values that were written to hardware. But it does mean that the following is idempotent: pwm_get_state(pwm, &state); pwm_apply_state(pwm, &state); Thierry
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