linux-kernel - Re: CoreSight framework and drivers

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Date:	Fri, 21 Dec 2012 14:17:12 -0800
From:	Pratik Patel <pratikp@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jon Hunter <jon-hunter@...com>, linux-arm-msm@...r.kernel.org,
	"linus.walleij@...aro.org" <linus.walleij@...aro.org>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org,
	"magnus.p.persson@...ricsson.com" <magnus.p.persson@...ricsson.com>,
	"david.rusling@...aro.org" <david.rusling@...aro.org>,
	"arve@...roid.com" <arve@...roid.com>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"d-deao@...com" <d-deao@...com>,
	"christian.bejram@...ricsson.com" <christian.bejram@...ricsson.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: CoreSight framework and drivers

On Thu, Dec 20, 2012 at 11:40:11PM +0000, Russell King - ARM Linux wrote:
> On Thu, Dec 20, 2012 at 04:54:38PM -0600, Jon Hunter wrote:
> > On 12/20/2012 01:51 PM, Pratik Patel wrote:
> > > Ok, so are you referring to making CoreSight devices register
> > > with AMBA bus instead of platform bus keeping everything else
> > > intact?
> > 
> > Yes exactly. However, please note I am not saying that we should do
> > this, and I asking what direction does the community want us to take
> > here? Platform bus or AMBA bus?
> 
> One of the issues which worries me about mixing peripheral drivers on
> random different buses is... what happens when we end up with a SoC
> which gates the APB clock at bus level (there are SoCs which gate the
> APB clock at peripheral level.)  In other words, an APB bus only gets
> clocked upon request.
> 
> We can deal with that with the infrastructure we have in place in the
> AMBA bus layer, but not with the platform bus - we'd have to teach the
> platform bus driver about the special APB clock instead of having it
> handled primerily at the bus layer.
> 
> At least the coresight ETM peripherals make use of the APB bus.  They
> have a whole pile of registers on the APB bus, and they have the
> primecell IDs stored in the last words of the peripheral, again just
> like the other primecell devices we have using the AMBA bus layer.
> 
> What I'd say is... why stick it on a different bus type from the other
> peripherals which might make things harder in the future?

Thanks for the info. I will look into using the AMBA bus instead
of the platform bus but one issue I notice is that AMBA framework
seems to support one contiguous register space.

CoreSight STM typically has a config register space and a
stimulus port/channel register space and these can be
non-contiguous in the chip memory map.

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
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