linux-kernel - Re: [PATCH 2/2] drivers/amba: probe via device tree

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Date:	Tue, 24 May 2011 10:03:35 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Grant Likely <grant.likely@...retlab.ca>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Kevin Hilman <khilman@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	devicetree-discuss@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] drivers/amba: probe via device tree

Grant,

On 05/23/2011 10:09 AM, Grant Likely wrote:
> On Mon, May 23, 2011 at 3:58 AM, Russell King - ARM Linux
> <linux@....linux.org.uk>  wrote:
>> On Mon, May 23, 2011 at 11:37:04AM +0200, Kristoffer Glembo wrote:
>>> Grant Likely wrote:
>>>> In the case we're talking about the bus really is an AMBA bus, and all
>>>> the devices on it are in some sense real amba devices.  The problem is
>>>> that not all of the devices on the bus implement peripheral ID
>>>> registers or other mechanisms that good upstanding AMBA devices are
>>>> expected to have.
>>>
>>> Before we go hardware bashing of non primecell AMBA devices I would just
>>> want to point out that the primecell stuff is not part of the AMBA
>>> specification.
>>
>> And before we go down that route, let me point out that the 'amba bus'
>> stuff in the kernel is there to support primecells, rather than all
>> devices which the AMBA specification covers.
>>
>> The reason it's called 'amba' is because back in 2001 or so when the
>> first primecell drivers were created, there was little information
>> available as to what AMBA, AHB, or APB even covered.  All I had to go
>> on were the primecell documents themselves.  The higher level documents
>> were not available to me.
>>
>> So, despite it being called 'amba', it really is just for primecells
>> and if we didn't have the exposure to userspace, I'd have renamed it to
>> 'apb' or similar instead.
>
> Okay, that clarifies things a lot, and lends weight to the arguement
> that it is perfectly normal and acceptable to have both amba_devices
> and platform_devices on the same bus segment.  Are there any cases
> where amba primecells are being driven by platform_drivers?  If so,
> should those drivers have an amba_driver registration added?

I would be surprised if there are any implemented as platform_drivers 
that are not duplicates of an amba driver. The STMP uart is actually a 
pl011 and it's platform driver was recently removed IIRC. So I think we 
can consider platform drivers something that should be fixed in this case.

Do you still think we should have a global match table of all devices or 
a generic "arm,primecell" compatible property would work. Several 
drivers like the pl022 have several h/w variations they support, so we 
would either need to list all those variations or have a generic name 
per device.

I think having "arm,amba-deviceid" is not needed. The current code does 
nothing but warn if it doesn't match the h/w value. The drivers already 
have a list of id's that they support and the amba bus only matches 
against the h/w id value. The only use I can see is overriding a broken 
h/w value. Certainly seems like it should be optional at least.

Rob
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