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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Message-ID: <20150528222412.GM30984@atomide.com>
Date:	Thu, 28 May 2015 15:24:13 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Matthijs van Duin <matthijsvanduin@...il.com>
Cc:	Pali Rohár <pali.rohar@...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)

* 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
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