[PATCH] doc: dm: clarify activation.

[Simon Glass]

Hi Michal,

On Thu, 4 Aug 2022 at 13:42, Michal Suchánek <msuchanek at suse.de> 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 <msuchanek at suse.de> wrote:
> > >
> > > Explain when devices should get activated.
> > >
> > > Signed-off-by: Michal Suchanek <msuchanek at suse.de>
> > > ---
> > >  doc/develop/driver-model/design.rst | 22 ++++++++++++++++++++--
> > >  1 file changed, 20 insertions(+), 2 deletions(-)
> >
> > Reviewed-by: Simon Glass <sjg at 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())
>
> 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