Hi Michal, On Thu, 4 Aug 2022 at 11:58, Michal Suchanek <msucha...@suse.de> wrote: > > Explain when devices should get activated. > > Signed-off-by: Michal Suchanek <msucha...@suse.de> > --- > doc/develop/driver-model/design.rst | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-)
Reviewed-by: Simon Glass <s...@chromium.org> Much appreciated! one note below > > diff --git a/doc/develop/driver-model/design.rst > b/doc/develop/driver-model/design.rst > index 5f33f9fbb3..c925d21b24 100644 > --- a/doc/develop/driver-model/design.rst > +++ b/doc/develop/driver-model/design.rst > @@ -794,8 +794,26 @@ fall afoul of this rule. > Activation/probe > ^^^^^^^^^^^^^^^^ > > -When a device needs to be used, U-Boot activates it, by first reading ofdata > -as above and then following these steps (see device_probe()): > +To save resources devices in U-Boot are probed lazily. U-Boot is a > bootloader, > +not an operating system. Many devices are never used during an U-Boot run, > and > +probing them takes time, requires memory, may add delays to the main loop, > etc. > + > +The device should be probed by the uclass code. Different uclasses are or the device code (e.g. device_get_child()) BTW probing a child probes all its parents automatically (that is said elsewhere I think). > +different but two common use cases can be seen: > + > + 1. The uclass is asked to look up a specific device, such as SPI bus 0, > + first chip select - in this case the returned device should be > + activated. > + > + 2. The uclass is asked to perform a specific function on any device that > + supports it, eg. reset the board using any sysreset that can be found - > + for this case the core uclass code provides iterators that activate > + each device before returning it, and the uclass typically implements a > + walk function that iterates over all devices of the uclass and tries > + to perform the requested function on each in turn until succesful. > + > +To activate a device U-Boot first reads ofdata as above and then follows > these > +steps (see device_probe()): > > 1. All parent devices are probed. It is not possible to activate a device > unless its predecessors (all the way up to the root device) are activated. > -- > 2.37.1 > Regards, Simon
