linux-kernel - Re: [PATCH v2 02/18] dt-bindings: timer: sp804: add timer-width property

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Date:	Tue, 22 Mar 2016 12:02:32 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	Neil Armstrong <narmstrong@...libre.com>,
	Rob Herring <robh@...nel.org>
Cc:	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sudeep Holla <sudeep.holla@....com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 02/18] dt-bindings: timer: sp804: add timer-width
 property

Hi Neil,

On 22/03/16 09:21, Neil Armstrong wrote:
> On 03/17/2016 08:21 PM, Robin Murphy wrote:
>> On 17/03/16 19:00, Rob Herring wrote:
>>> On Thu, Mar 17, 2016 at 1:06 PM, Robin Murphy <robin.murphy@....com> wrote:
>>>> Hi Rob,
>>>>
>>>> On 17/03/16 17:09, Rob Herring wrote:
>>>>> That would not be an SP804 nor would the vendor be ARM in that case. So
>>>>> add a new compatible string for the vendor that decided to hack up ARM's
>>>>> IP block.
>>>>
>>>>
>>>> By all accounts this is some ancient reference design[1] which later evolved
>>>> _into_ the SP804, so that vendor would probably still be ARM ;)
>>>
>>> Right.
>>>
>>>> A separate compatible string would indeed make more sense, though. Both
>>>> semantically and in terms of letting the driver account for the differences
>>>> automatically.
>>>>
>>>> Robin.
>>>>
>>>> [1]:http://infocenter.arm.com/help/topic/com.arm.doc.ddi0170a/I350250.html
>>>
>>> Humm, same as integrator timers perhaps?
>>
>> Having had a quick look, what the Integrator/AP manual describes certainly smells like the same basic block as the "AMBA Timer" - 16 bit counters and the same control register layout - albeit in a mutant triple-timer version with a bigger offset between each register set. Integrator/CP, on the other hand, looks much more SP804-like.
>>
>> Robin.
>>
>
> Hi,
>
> I will switch to oxsemi,ox810se-rps-timer since it need a specific register width that will be handled by the driver.

By "needs a specific register width" do you mean the OxSemi 
implementation will give a bus error on a 32-bit access and requires 
16-bit accessors? If so, I'd expect to see patch 1 changing readl()s to 
readw()s at least somewhere. Otherwise, if it's merely that the 
clocksource API needs to know the upper 16 bits of a word it reads are 
undefined, then since that's the standard behaviour I'd be inclined to 
add it to the driver as a canonical "arm,amba-timer" implementation, 
then have your implementation-specific compatible on top of that just in 
case.

Robin.

>
> Thanks,
> Neil
>
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