On 8/4/22 22:30, Simon Glass wrote:
Hi Michal,
On Thu, 4 Aug 2022 at 13:42, Michal Suchánek <msucha...@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 <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
When merging I will put the following here:
The device should be probed by the uclass code or generic device code
(e.g. device_find_global_by_ofnode()). Uclasses differ but two common
use cases can be seen:
Best regards
Heinrich
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
