Hi Michal, On Thu, 4 Aug 2022 at 13:42, Michal Suchánek <[email protected]> wrote: > > On Thu, Aug 04, 2022 at 01:22:53PM -0600, Simon Glass wrote: > > Hi Michal, > > > > On Thu, 4 Aug 2022 at 11:58, Michal Suchanek <[email protected]> wrote: > > > > > > Explain when devices should get activated. > > > > > > Signed-off-by: Michal Suchanek <[email protected]> > > > --- > > > doc/develop/driver-model/design.rst | 22 ++++++++++++++++++++-- > > > 1 file changed, 20 insertions(+), 2 deletions(-) > > > > Reviewed-by: Simon Glass <[email protected]> > > > > 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()) > > Can you elaborate a bit more, please?
I mean that we can use 'device' code to probe, as well as the 'uclass' code. The function I cite is an example of that, but actually a better one is device_find_global_by_ofnode() > > This change is the reasult of some previous discussion about the device > activation with Marek. > > The device activation is really difficult to understand for people who > did not participate in designing the DM, and is probably quite specific > to U-Boot. OK, well good to have it. I wish more people would do what you have done here, i.e. go back and update the docs to clarify things that you found confusing / inadequate. > > > > > BTW probing a child probes all its parents automatically (that is said > > elsewhere I think). > > Yes, right below this part. > > Thanks > > Michal OK Regards, Simon > > > > > > +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
