On Fri, 2017-04-21 at 16:55 +0200, Peter Rosin wrote: > On 2017-04-21 16:41, Philipp Zabel wrote: > > On Fri, 2017-04-21 at 16:32 +0200, Peter Rosin wrote: > >> On 2017-04-21 16:23, Philipp Zabel wrote: > >>> On Thu, 2017-04-13 at 18:43 +0200, Peter Rosin wrote: > >>> [...] > >>>> +int mux_chip_register(struct mux_chip *mux_chip) > >>>> +{ > >>>> + int i; > >>>> + int ret; > >>>> + > >>>> + for (i = 0; i < mux_chip->controllers; ++i) { > >>>> + struct mux_control *mux = &mux_chip->mux[i]; > >>>> + > >>>> + if (mux->idle_state == mux->cached_state) > >>>> + continue; > >>> > >>> I think this should be changed to > >>> > >>> - if (mux->idle_state == mux->cached_state) > >>> + if (mux->idle_state == mux->cached_state || > >>> + mux->idle_state == MUX_IDLE_AS_IS) > >>> continue; > >>> > >>> or the following mux_control_set will be called with state == > >>> MUX_IDLE_AS_IS. Alternatively, mux_control_set should return when passed > >>> this value. > >> > >> That cannot happen because ->cached_state is initialized to -1 > >> in mux_chip_alloc, so should always be == MUX_IDLE_AS_IS when > >> registering. And drivers are not supposed to touch ->cached_state. > >> I.e., ->cached_state is "owned" by the core. > > > > So this was caused by me filling cached_state from register reads in the > > mmio driver. Makes me wonder why I am not allowed to do this, though, if > > I am able to read back the initial state? > > You gain fairly little by reading back the original state. If the mux > should idle-as-is, you can avoid a maximum of one mux update if the first > consumer happens to starts by requesting the previously active state. > Similarly, if the mux should idle in a specific state, you can avoid a > maximum of one mux update. > > In both cases it costs one unconditional read of the mux state. > > Sure, in some cases reads are cheaper than writes, but I didn't think > support for seeding the cache was worth it. Is it worth it? Probably not, I'll just drop the cached_state initialization. It should be documented in the mux.h that this field is framework internal and not to be touched by the drivers. At least I was surprised. regards Philipp -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html