[PATCH] doc: dm: clarify activation.

[Michal Suchánek]

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?

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.

> 
> BTW probing a child probes all its parents automatically (that is said
> elsewhere I think).

Yes, right below this part.

Thanks

Michal

> 
> > +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