[PATCH 3/4] doc: driver-model: state that of_to_plat() must not probe or claim

[Mehmet Fide <mehmet.fide@gmail.com>]

From: Mehmet Fide <mehmet.fide@screeningeagle.com>

The design document says decoding the devicetree belongs in
of_to_plat(), but not what the method must not do. Spell out the rule
that has always been implied by the phase separation: no probing of
other devices and no claiming of resources, and point at the
parse-now-request-later GPIO helpers as the pattern to follow.

Signed-off-by: Mehmet Fide <mehmet.fide@screeningeagle.com>
---
 doc/develop/driver-model/design.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/doc/develop/driver-model/design.rst b/doc/develop/driver-model/design.rst
index 633545944d1..7f60c75f4d5 100644
--- a/doc/develop/driver-model/design.rst
+++ b/doc/develop/driver-model/design.rst
@@ -759,6 +759,15 @@ The steps are:
 
    6. The device is marked 'plat valid'.
 
+The of_to_plat() method must only read the devicetree. It must not probe
+other devices or claim resources such as GPIOs or clocks: ofdata is read
+before the device's pinctrl state is applied, so a pin claimed here can have
+its configuration undone a moment later, and probing another device from this
+method defeats the lazy-probing model. When a resource is named in the
+devicetree, read its description into the platform data here - for a GPIO,
+gpio_parse_by_name() - and claim it in probe(), for a GPIO with
+gpio_request_parsed().
+
 Note that ofdata reading is always done (for a child and all its parents)
 before probing starts. Thus devices go through two distinct states when
 probing: reading platform data and actually touching the hardware to bring
-- 
2.54.0