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Subject: Re: [PATCH v0] irqchip/gic-v3: Avoid check of lpi configuration for
 non existent cpu
To:     Marc Zyngier <maz@kernel.org>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org
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From:   Gaurav Kohli <gkohli@codeaurora.org>
Message-ID: <0101016eda62405c-0df52650-b61f-4ad6-a1df-2eed055e1984-000000@us-west-2.amazonses.com>
Date:   Fri, 6 Dec 2019 08:45:03 +0000
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On 12/5/2019 6:48 PM, Marc Zyngier wrote:
> On 2019-12-05 13:01, Gaurav Kohli wrote:
>> On 12/5/2019 6:17 PM, Marc Zyngier wrote:
>>> Hi Gaurav,
>>> On 2019-12-05 10:55, Gaurav Kohli wrote:
>>>> As per GIC specification, we can configure gic for more no of cpus
>>>> then the available cpus in the soc, But this can cause mem abort
>>>> while iterating lpi region for non existent cpu as we don't map
>>> Which LPI region? We're talking about RDs, right... Or does LPI mean
>>> something other than GIC LPIs for you?
>>>
>>
>> Yes RDs only.
>>>> redistrubutor region for non-existent cpu.
>>>>
>>>> To avoid this issue, put one more check of valid mpidr.
>>> Sorry, but I'm not sure I grasp your problem. Let me try and rephrase 
>>> it:
>>> - Your GIC is configured for (let's say) 8 CPUs, and your SoC has 
>>> only 4.
>> Yes, suppose gic is configured for 8 cpus but soc has only 4 cpus.
>> Then in this case gic_iterate will iterate till it get TYPER_LAST.
> 
> And that's what is expected from the architecture.
> 
>>
>> But as gic is configured for 8, So last bit sets in eight
>> redistributor regions only.
>>> - As part of the probing, the driver iterates on the RD regions and 
>>> explodes
>>>    because something isn't mapped?
>>> That'd be a grave bug, but I believe the issue is somewhere else.
>>
>> There are 4 cpus present, that's why we have mapped 4 redistributor
>> only, but during probe below function keeps iterating and give mem
>> abort for 5th cpu.
>>
>> static void gic_update_vlpi_properties(void)
>> {
>>         gic_iterate_rdists(__gic_update_vlpi_properties);
>>
>> }
>>
>> We can solve this problem by mapping all eight redistributor in dt,
>> but ideally code should also able to handle this and we can avoid
>> mappin?
> 
> The whole point of DT is to describe the HW, all the HW, nothing but
> the HW. This is what is expected by both the architecture and Linux.
> 
> So you have the solution already. Don't lie to the kernel, and everything
> will be fine.
> 
>          M.

HI Marc,

Thanks for detailed explanation, Yes we have mapped all 8 distributors 
now to resolve.
But my main concern is that last 4 redistributor is not connected to 
core, as core is not present.
And as per gic driver it seems we are only
iterating and populating per cpu rd pointer.

So that would be fine correct, seems nothing wrong in this kind of 
configuration?

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