linux-kernel - Re: [PATCH v3 03/11] drivers: base: add new class "cpu" to group cpu devices

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Message-ID: <53F60808.10707@arm.com>
Date:	Thu, 21 Aug 2014 15:54:00 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	David Herrmann <dh.herrmann@...il.com>
CC:	Sudeep Holla <sudeep.holla@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH v3 03/11] drivers: base: add new class "cpu" to group
 cpu devices



On 21/08/14 13:37, David Herrmann wrote:
> Hi
>
> On Thu, Aug 21, 2014 at 2:30 PM, Sudeep Holla <sudeep.holla@....com> wrote:
>> On 21/08/14 12:20, David Herrmann wrote:
>>> Why do we need a cpu-class if the same set of information is already
>>> available on the cpu-bus? Furthermore, classes are deprecated anyway.
>>> Everything you can do with a class can be solved with a bus. And we
>>> already have a bus for cpus.
>>>

Sorry I wasn't aware that classes are deprecated. IIUC its class_device
struct and related APIs that are deprecated not struct class. I may be
wrong but Greg would not have suggested to use class if it was deprecated.

>>
>> This was suggested[1] by GregKH. The main reason it was added is to
>> reuse the device attributes rather than creating the raw kobjects.
>>
>> It helps to move few other cpu related subsystems using raw kobjects to
>> the device attribute groups.
>
> So the only reason to add a class is to get attributes registered
> properly with the device? Just set dev->groups before calling
> device_add()? This works just fine on buses, too.
>

Do you mean to say create bus for each cpu and add cpu devices like
cache under that ? We can't reuse the cpu bus i.e cpu_subsys as we can't
add per-cpu devices with same name under /sys/bus/cpu/devices/

Also adding bus for each cpu device might be more complex than creating
cpu class.

That's the reason why I added checks to avoid creating link in the class
directory pointing to the device. It would be difficult to do
that with bus as it can't be unconditional.

Regards,
Sudeep


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