linux-kernel - Re: [PATCH 2/2] irqchip/gic-v2m: Add workaround for Broadcom NS2 GICv2m erratum

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Date:	Wed, 4 May 2016 09:20:51 -0700
From:	Ray Jui <ray.jui@...adcom.com>
To:	Marc Zyngier <marc.zyngier@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>
Cc:	linux-kernel@...r.kernel.org,
	bcm-kernel-feedback-list@...adcom.com,
	linux-arm-kernel@...ts.infradead.org,
	Alex Barba <alex.barba@...adcom.com>
Subject: Re: [PATCH 2/2] irqchip/gic-v2m: Add workaround for Broadcom NS2
 GICv2m erratum

Hi Marc,

On 5/4/2016 12:49 AM, Marc Zyngier wrote:
> On 04/05/16 00:47, Ray Jui wrote:
>> Alex Barba <alex.barba@...adcom.com> discovered Broadcom NS2 GICv2m
>> implementation has an erratum where the MSI data needs to be the SPI
>> number subtracted by an offset of 32, for the correct MSI interrupt to
>> be triggered.
>>
>> We are aware that APM X-Gene GICv2m has a similar erratum where the
>> MSI data needs to be the offset from the spi_start. While APM's workaround
>> is triggered based on readings from the MSI_IIDR register, this patch
>> contains a more general solution by allowing this offset to be
>> specified with an optional DT property 'arm,msi-offset-spi'. This patch
>> also maintains compatibility with existing APM platforms
>
> It may be more generic, but it also fails to deal with less capable
> firmware implementations. In contrast, reading MSI_IIDR is always
> possible (assuming you have a unique ID for this v2m implementation).
>
> If you cannot uniquely identify it using an ID register, the usual
> alternative is to have a new "compatible" string identifying the
> defective part, and set the offset based on this string. This still
> fails the ACPI test, but is the least invasive DT-wise.

Okay. We do seem to have an ID. The JEP code looks a bit weird as the 
IIDR register reads 0x13f. We were just a bit concerned that there's 
another chip from Broadcom that may happen to have the same ID but may 
already have this offset issue fixed (or made worse with a different 
offset, :) ). Since that chip has not even taped out yet, we can wait 
till later to confirm. If a compatible string is needed in the future, 
we'll add that.

For now, I'm going to submit another patch to deal with this offset 
based on IIDR reading 0x13f.

>
> Thanks,
>
> 	M.
>

Thanks,

Ray