linux-kernel - Re: Could I (ab)use bus (struct bus

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Date:	Wed, 20 Apr 2011 09:29:53 +0200
From:	Rafał Miłecki <zajec5@...il.com>
To:	Arend van Spriel <arend@...adcom.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	George Kashperko <george@...u.edu.ua>,
	Hauke Mehrtens <hauke@...ke-m.de>,
	Russell King <rmk@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jonas Gorski <jonas.gorski@...il.com>,
	"b43-dev@...ts.infradead.org" <b43-dev@...ts.infradead.org>,
	Greg KH <greg@...ah.com>, Andy Botting <andy@...ybotting.com>,
	Larry Finger <Larry.Finger@...inger.net>
Subject: Re: Could I (ab)use bus (struct bus_type) for virtual Broadcom bus?

2011/4/20 Arend van Spriel <arend@...adcom.com>:
> On Tue, 19 Apr 2011 16:35:40 +0200, Arnd Bergmann <arnd@...db.de> wrote:
>
>> On Tuesday 19 April 2011, Arend van Spriel wrote:
>>
>>> > A new bus_type really only makes sense if you expect a lot of devices
>>> > to use this and you want to have the probing in the bus. If you only
>>> > want to have a way to enumerate devices that get created by the
>>> > parent driver, you can also use platform devices.
>>>
>>> The main assumption of the (bcm)axi driver seems to be that each core can
>>> be considered as a device. Correct me if I am wrong, but I consider a
>>> device to be an entity providing a particular system function. So an
>>> ethernet device provides ethernet connectivity function, a mixer device
>>> provides sound mixing function, and so on. The cores within a chip are
>>> not
>>> always self-contained like this. To clarify let's say a system function
>>> is
>>> realized by programming core A, core B, and finally trigger core A to set
>>> the function in motion. This implies the need of coordination between the
>>> programming steps on those cores.
>>>
>>> Is my view on what is a device wrong? Does a platform device differ in
>>> this respect from a regular device?
>>
>> A platform device means something that cannot be probed, in every other
>> respect it is the same as other devices. From your explanation above,
>> it seems that you don't actually need to represent the cores on your
>> particular chips as struct device in Linux at all.
>>
>> If you wanted to use platform_device, the right way would probably
>> be an MFD device that creates multiple child devices (which end
>> up as platform_device, though you don't need to worry about that)
>> from the PCI driver. Then you could use the child devices completely
>> independent from one another.
>>
>> Since you say that the cores in this case are tightly coupled and
>> don't provide independent functionality to the system, there is
>> no need to represent them as devices.
>
> Hi Arnd,
>
> The case is a hypothetical one, but I consider it a likely one. The axi bus
> driver currently registers each detected core as a device in the linux
> device tree. My statement is that this implies loose or no coupling between
> those cores, which is not true. One (or two) exceptions have already been
> identified. I would expect your last line to read: ...to the system, those
> cores should not be represented as devices.

I do not register that exceptional devices in my code.

-- 
Rafał
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