On Sat, Dec 06, 2014 at 09:08:36PM +0900, Alexandre Courbot wrote: > On Fri, Dec 5, 2014 at 7:24 PM, Maxime Ripard > <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > > On Thu, Dec 04, 2014 at 11:49:19PM +0900, Alexandre Courbot wrote: > >> On Thu, Dec 4, 2014 at 11:27 PM, Maxime Ripard > >> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > >> > Hi, > >> > > >> > On Thu, Dec 04, 2014 at 11:15:38PM +0900, Alexandre Courbot wrote: > >> >> On Wed, Dec 3, 2014 at 1:12 AM, Maxime Ripard > >> >> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > >> >> > On Tue, Dec 02, 2014 at 03:29:46PM +0100, Linus Walleij wrote: > >> >> >> On Tue, Dec 2, 2014 at 3:13 PM, Alexandre Courbot <gnurou@xxxxxxxxx> wrote: > >> >> >> > On Tue, Dec 2, 2014 at 1:36 AM, Maxime Ripard > >> >> >> > <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > >> >> >> > >> >> >> >> The only thing I'd like to have would be that the request here would > >> >> >> >> be non-exclusive, so that a later driver would still be allowed later > >> >> >> >> on to request that GPIO later on and manage it itself (ideally using > >> >> >> >> the usual gpiod_request function). > >> >> >> > > >> >> >> > Actually we have a plan (and I have some code too) to allow multiple > >> >> >> > consumers per GPIO. Although like Benoit I wonder why you would want > >> >> >> > to hog a GPIO and then request it properly later. Also, that probably > >> >> >> > means we should abandon the hog since it actively drives the line and > >> >> >> > would interfere with the late requested. How to do that correctly is > >> >> >> > not really clear to me. > >> >> >> > >> >> >> I don't get the usecase. A hogged GPIO is per definition hogged. > >> >> >> This sounds more like "initial settings" or something, which is another > >> >> >> usecase altogether. > >> >> > > >> >> > We do have one board where we have a pin (let's say GPIO14 of the bank > >> >> > A) that enables a regulator that will provide VCC the bank B. > >> >> > > >> >> > Now, both banks are handled by the same driver, but in order to have a > >> >> > working output on the bank B, we do need to set GPIO14 as soon as > >> >> > we're probed. > >> >> > > >> >> > Just relying on the usual deferred probing introduces a circular > >> >> > dependency between the gpio-regulator that needs to grab its GPIO from > >> >> > a driver not there yet, and the gpio driver that needs to enable its > >> >> > gpio-regulator. > >> >> > >> >> I don't get it. According to what you said, the following order should > >> >> go through IIUC: > >> >> > >> >> 1) bank A is probed, gpio 14 is available > >> >> 2) gpio-regulator is probed, acquires GPIO 14, regulator for Bank B is available > >> >> 3) bank B is probed, grabs its regulator and turn it on, probes. > >> >> > >> >> What am I missing? > >> > > >> > It would be true if bank A and B were exposed through different > >> > drivers (or at least different instances of the same driver), which is > >> > not the case. > >> > > >> > In our case, banks A and B are handled by the same instance. > >> > >> Ok, so both banks A and B are part of the same device/DT node. Now I > >> think I understand the issue. You need to hog the pin so that bank B > >> will work right after the device is probed. > > > > Exactly. > > > >> But you will still have the problem that the regulator device will > >> *not* be available when your device is probed, so you cannot call > >> regulator_get() for bank B anyway. I guess your only choice is to hog > >> that pin and leave it active ad vitam eternam. > > > > Hmmm, indeed. > > > > I'll stop boring you with this then :) > > Please *keep* bothering us with any doubt you may have until they are > all cleared and you are sure this feature will be useful to you. > Especially since we are designing DT bindings that will have to be > carried over forever. We really want to get them right, and need input > of potential users for that. > > Having a few design arguments is a small thing compared to the hassle > of having to work with unadapted features and bindings. Ok. What I had in mind when I first thought about it was to set GPIO as hogged through the GPIO flags, and then have a dumb GPIO driver like Pantelis was suggesting. I don't know if hogged would still be the right term, but we could have that flag that would just allow the value to be set through gpio_request init value, and deny/cache any subsequent change through gpio_set_value. gpio_request with this flag would never return EPROBE_DEFER, and just cache the value to be set for when the driver comes in. We could enforce driver-less GPIOs through that dumb driver, and we would still be able to break weird dependency chains that end up in circle. That's just a thought though. Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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