linux-kernel - Re: [RFC] driver-core: Remove dummy 'platform

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Date:	Wed, 23 Apr 2014 15:44:10 +0100
From:	Grant Likely <grant.likely@...aro.org>
To:	Rob Herring <robherring2@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [RFC] driver-core: Remove dummy 'platform_bus'

On Wed, Apr 23, 2014 at 3:05 PM, Grant Likely <grant.likely@...aro.org> wrote:
> On Mon, 21 Apr 2014 16:05:29 -0500, Rob Herring <robherring2@...il.com> wrote:
>> On Wed, Nov 21, 2012 at 8:44 AM, Grant Likely <grant.likely@...retlab.ca> wrote:
>> > The "platform_bus" (note: not platform_bus_type) only exists as an empty
>> > directory to put platform devices into. However, it really doesn't make
>> > sense to segregate all the platform devices into a sub directory when
>> > typically they are memory mapped devices that doen't go through any
>> > particular bus. Particularly on embedded type platforms the platform_bus
>> > directory doesn't add anything.
>> >
>> > However, this will probably just end up breaking some userspace that
>> > depends on the /sys/devices/platform/ path to be present (no matter how
>> > much we protest that userspace must not depend on paths in sysfs). So
>> > while I'm seriously proposing this change, it may just be unacceptable
>> > ABI breakage
>>
>> An old thread, but was there ever a conclusion to this? We now have a
>> mixture of using platform_bus as the parent or not on various ARM
>> platforms.
>
> We kind of concluded in the opposite direction. Instead of removing the
> /sys/device/platform directory, the drivers/of code should be changed to
> use it.
>
> The following patch is sufficient to have the same effect. It doesn't
> unify the OF and non-OF paths of platform device addition, but it gets
> them closer. I've been nervous about applying it because I'm concerned
> about userspace breakage, but maybe it just needs to be merged and we
> can quirk out systems that break.

Ugh, no matter what we do, something is going to be inconsistent. With
this patch, the platform_devices get moved under
/sys/devices/platform, but an amba_device at the top level stays where
it is. We could move amba devices under the same hierarchy, but that
doesn't seem appropriate.

We could have a separate /sys/devices/of hierarchy for generating
devices into if this really is a problem that needs to be solved. I'm
not convinced that a real problem exists though. PowerPC has been
creating the devices directly under /sys/devices at least since
2.6.12. With the DT being hierarchical to begin with, and the dt code
preserving the hierarchy, on most platforms there won't be a lot of
things at the root.

Anyway, whatever. I don't care enough to argue anymore. Move them if
you like, but make sure the root AMBA devices get created in the same
place.

g.

>
> ---
>
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index 404d1daebefa..40a85b85c932 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -175,7 +175,7 @@ struct platform_device *of_device_alloc(struct device_node *np,
>  #if defined(CONFIG_MICROBLAZE)
>         dev->dev.dma_mask = &dev->archdata.dma_mask;
>  #endif
> -       dev->dev.parent = parent;
> +       dev->dev.parent = parent ? parent : &platform_bus;
>
>         if (bus_id)
>                 dev_set_name(&dev->dev, "%s", bus_id);
>
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