On Wed, 2019-10-30 at 15:32 +0200, Peter Ujfalusi wrote: > > On 30/10/2019 15.12, Rob Herring wrote: > > On Wed, Oct 30, 2019 at 7:03 AM Peter Ujfalusi <peter.ujfalusi@xxxxxx> wrote: > > > Hi, > > > > > > The shared GPIO line for external components tends to be a common issue and > > > there is no 'clean' way of handling it. > > > > > > I'm aware of the GPIOD_FLAGS_BIT_NONEXCLUSIVE flag, which must be provided when > > > a driver tries to request a GPIO which is already in use. > > > However the driver must know that the component is going to be used in such a > > > way, which can be said to any external components with GPIO line, so in theory > > > all drivers must set this flag when requesting the GPIO... > > > > > > But with the GPIOD_FLAGS_BIT_NONEXCLUSIVE all clients have full control of the > > > GPIO line. For example any device using the same GPIO as reset/enable line can > > > reset/enable other devices, which is not something the other device might like > > > or can handle. > > > For example a device needs to be configured after it is enabled, but some other > > > driver would reset it while handling the same GPIO -> the device is not > > > operational anymmore as it lost it's configuration. > > > > > > With the gpio-shared gpiochip we can overcome this by giving the gpio-shared > > > the role of making sure that the GPIO line only changes state when it will not > > > disturb any of the clients sharing the same GPIO line. > > > > Why can't we just add a shared flag like we have for interrupts? > > Effectively, we have that for resets too, it's just hardcoded in the > > the drivers. > > This would be kind of the same thing what the > GPIOD_FLAGS_BIT_NONEXCLUSIVE does, which was a quick workaround for > fixed-regulators afaik. > > But let's say that a board design will pick two components (C1 and C2) > and use the same GPIO line to enable them. We already have the drivers > for them and they are used in boards already. > > Both needs the GPIO line to be high for normal operation. > One or both of them needs register writes after they are enabled. > > During boot both requests the GPIO (OUTPUT_LOW) and sets it high, then > run the register setup. > > C1 request GPIO (LOW) > C1 gpio_set(1) > C1 register writes > C2 requests GPIO (LOW) > C1 placed to reset and looses the configuration > C2 gpio_set(1) > C1 also enabled > C2 register writes > > At this point C2 is operational, C1 is not. > > In shared GPIO case the GPIO should be handled like a regulator with a > twist that the 'sticky' state of the GPIO might be low or high depending > on the needs of the components it is connected to. > > The shared GPIO line is a board design quirk and basically any device > which have reset/enable GPIO must be able to work in a situation when > they are sharing that line with other components and the driver should > not know much about this small detail. What about components that require a register write right after being enabled, for example to put the device into a low power state, to silence it on a bus, or to mask some initially enabled interrupts? regards Philipp
