Re: [PATCH RFC] of: platform: Drop OF_POPULATED_BUS check from of_platform_notify()

Hello, 

On 08/09/16 16:17, Rob Herring wrote:

quoted

The comment to OF_POPULATED_BUS says "of_platform_populate recursed to children

quoted

of this node". Consider the following structure:

        ocp {
                compatible = "simple-bus";
                ranges;
                ...
                l4_wkup: l4_wkup@44c00000 {
                        compatible = "ti,am3-l4-wkup", "simple-bus";
                        ranges = <0 0x44c00000 0x280000>;
                        ...
                        wkup_m3: wkup_m3@100000 {
                                compatible = "ti,am3352-wkup-m3";
                                ...
                        }
                }
        }

If one DT overlay adds "ocp" node without children then of_platform_notify()
creates a platform device, but doesn't set OF_POPULATED_BUS. If second overlay
tries to populate children of "ocp" node, corresponding platform devices are
not instantiated because of check for OF_POPULATED_BUS flag. If the above
example structure is built of several overlays, of_platform_populate() is not
involved here at all, so maybe OF_POPULATED_BUS requirement is wrong.

Your example seems a bit made up. While you can craft base DTs and
overlays with any random split between them, I don't necessarily think
we want to support that. There will likely be cases which we say can
only be applied early in boot or we limit which nodes can be overlay
targets for overlays applied from userspace.

Yes, I had to construct an example out of the well-known Beaglebone DT pieces,
the real use case will only confuse the public.

In general I don't understand why my example is so bizarre, it basically shows, you
cannot apply an overlay above overlay: the devices will not be registered and actually
because of the bogus check. Imagine the well known Beagle: you can construct a bone with
whatever SPI/I2C bus controller. On top of it you can sandwich a board with I2C-driven
GPIO extenders. Is it absolute unrealistic? 

There is full support of multilevel overlays. And just one bug preventing it from working
properly. Do we really want it?

-- 
Best regards,
Alexander Sverdlin.
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