linux-kernel - Re: [PATCH] irqchip: gic: Fix core ID calculation when topology is read from DT

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Date:	Thu, 17 Jul 2014 11:51:05 -0400
From:	Jason Cooper <jason@...edaemon.net>
To:	Tomasz Figa <t.figa@...sung.com>
Cc:	linux-samsung-soc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Kukjin Kim <kgene.kim@...sung.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH] irqchip: gic: Fix core ID calculation when topology is
 read from DT

On Thu, Jul 17, 2014 at 05:40:50PM +0200, Tomasz Figa wrote:
> Hi Jason,
> 
> On 17.07.2014 17:32, Jason Cooper wrote:
> > On Thu, Jul 17, 2014 at 05:23:44PM +0200, Tomasz Figa wrote:
> >> Certain GIC implementation, namely those found on earlier, single
> >> cluster, Exynos SoCs, have registers mapped without per-CPU banking,
> >> which means that the driver needs to use different offset for each CPU.
> >>
> >> Currently the driver calculates the offset by multiplying value returned
> >> by cpu_logical_map() by CPU offset parsed from DT. This is correct when
> >> CPU topology is not specified in DT and aforementioned function returns
> >> core ID alone. However when DT contains CPU topology, the function
> >> changes to return cluster ID as well, which is non-zero on mentioned
> >> SoCs and so breaks the calculation in GIC driver.
> >>
> >> This patch fixes this by masking out cluster ID in CPU offset
> >> calculation so that only core ID is considered. Multi-cluster Exynos
> >> SoCs already have banked GIC implementations, so this simple fix should
> >> be enough.
> >>
> >> Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> >> Reported-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
> >> Signed-off-by: Tomasz Figa <t.figa@...sung.com>
> >> ---
> >>  drivers/irqchip/irq-gic.c | 5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > iiuc, this was introduced by:
> > 
> >   db0d4db22a78d ARM: gic: allow GIC to support non-banked setups
> > 
> > and so should be for v3.3 and up, correct?
> 
> Could be, although there was and still is no topology data specified in
> DT for affected Exynos SoCs. The need for it showed up just recently, so
> I'm not sure this is a regression to fix in older kernels.

In my "the kernel and the dtb aren't tied together" quest, these are the
kinds of things I like to see fixed in stable kernels.

If a user needs to update a dtb, say to fix a bug, it's reasonable to
use the newest one for a given board.  After all, any new nodes won't
change anything, since the driver in the kernel won't match the node.

However, in this case, without this fix, a user upgrading to the newest
dtb would get a broken system.  So, this fix should be backported to
prevent the breakage.  Or, have I missed something in my analysis?

thx,

Jason.
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