linux-kernel - Re: runtime check for omap-aes bus access permission (was: Re: 3.13-rc3 (commit 7ce93f3) breaks Nokia N900 DT boot)

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Date:	Fri, 29 May 2015 00:27:21 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Tony Lindgren <tony@...mide.com>
Cc:	Matthijs van Duin <matthijsvanduin@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Sebastian Reichel <sre@...g0.de>,
	"linux-omap" <linux-omap@...r.kernel.org>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	Pavel Machek <pavel@....cz>,
	lkml <linux-kernel@...r.kernel.org>, Nishanth Menon <nm@...com>
Subject: Re: runtime check for omap-aes bus access permission (was: Re: 3.13-rc3 (commit 7ce93f3) breaks Nokia N900 DT boot)

On Friday 29 May 2015 00:24:13 Tony Lindgren wrote:
> * Matthijs van Duin <matthijsvanduin@...il.com> [150528 13:28]:
> > On 28 May 2015 at 18:01, Tony Lindgren <tony@...mide.com> wrote:
> > > For failed device access you get an interrupt
> > 
> > Well for failed reads you get a bus error, and "catching" those
> > (e.g. using the existing exception mechanism used to catch MMU
> > faults) is the whole issue.
> > 
> > Though now that you mention it, it is true that for writes you
> > won't get any fault (at least on the DM814x and AM335x the posting
> > point appears to be the async bridge from MPUSS to the L3
> > interconnect) but an interconnect error irq instead. It may be
> > easier to make some kind of harmless write (e.g. to the version
> > register), wait a bit, and check if the write triggered an
> > interconnect error.
> > 
> > Feels hackish though: you'd need to be sure you waited long enough
> > (though using a read from another device on the same L4
> > interconnect should be a reliable barrier in this case), and
> > drivers for receiving/interpreting interconnect errors are not
> > implemented yet on all SoCs (for some, like the AM335x, TI didn't
> > even bother publishing the relevant data in its TRM). Interconnect
> > errors can also be lost in some cases (multiple errors involving
> > the same target in a short time window) though that problem
> > shouldn't arise in this particular case.
> 
> Hmm I believe the interrupt happens immediately trying to access an
> invalid device. But maybe I'm thinking about just errors if a device
> is not powered or clocked. So obviously some experiments need to be
> done :)
> 
> The advantage here would be that the l3 driver actually already knows
> quite a bit about the devices on the bus.
> 
> > Also, presumably interconnect error reporting is unavailable on HS
> > devices given the fact that all interconnect registers seemed to be
> > inaccessible?
> 
> Oh OK yeah then that would not work for Pali's case. I guess it just
> needs to be tested.
> 
> Regards,
> 
> Tony

Ok, thanks for info. Do you have some quick small patches for testing? 
Or some pointers what is needed to modify and how?

-- 
Pali Rohár
pali.rohar@...il.com

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