On Thu, Dec 4, 2014 at 11:27 PM, Pantelis Antoniou <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote: > Hi Alexandre, > > I tried to stay away while things are being fleshed out but… > >> On Dec 4, 2014, at 16:15 , Alexandre Courbot <gnurou@xxxxxxxxx> 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? >> >>> >>> GPIO hogging needs to be the ideal solution for that, since we can >>> just enforce the GPIO14 value as the driver is probed, which provides >>> the guarantee that any driver using the bank B will actually drive the >>> GPIO it might use. >> >> At this point I start wondering if such initial setup should not be >> the job of the bootloader? GPIO hogging ought to be simple and >> definitive, adding the possibility to have it just as an initial value >> would considerably complexify it. E.g. when is the gpio chip driver >> supposed to release the hogged descriptor in such a case? >> > > Do not count on the bootloader setting up anything. The trend is > for the bootloader to setup the minimal environment to load your kernel > and jump to it. > > http://www.denx.de/wiki/pub/U-Boot/MiniSummitELCE2013/2013-ELCE-U-Boot-Falcon-Boot.pdf Just wondering. :) But yeah, there are some use-cases (such as this one or Linux-as-a-bootloader) for which this would not play nicely. > > >> Note that if the multiple GPIO consumer feature we are planning goes >> through, you should be able to use both hogging *and* a regulator on >> the same GPIO and achieve what you want. The expectation of multiple >> consumers is that the board designers know what they are doing, and >> this case would certainly fit (chip hogs the line and doesn't touch >> the value after that, letting the regulator control it without any >> conflict afterwards), although it would of course be better to solve >> the issue through regular probing... > > > That’s why I was advocating a simple probing driver for all this. > Figure out a way for this driver to be probed first would be an easier > solution that what’s going on here. Do you mean, a driver whose sole job is to probe other drivers in the right order? :/ -- 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
