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 17:58:33 +0200
From:	Tomasz Figa <t.figa@...sung.com>
To:	Jason Cooper <jason@...edaemon.net>
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 17.07.2014 17:51, Jason Cooper wrote:
> 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?

This is correct when looking only at this issue. However I suspect such
move would cause a breakage anyway, because DT stuff isn't that stable
on Exynos side. I don't mind if this patch hits stable, though.

Best regards,
Tomasz
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