linux-kernel - Re: [PATCH RFC 1/1] iommu/arm-smmu: Fix context fault message considering non-NTS

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Message-ID: <CAFp+6iENb9rf5-Mz5m41roo9DHasb6=TtWDPxjv2qHGSez1E+g@mail.gmail.com>
Date:   Tue, 6 Feb 2018 18:17:43 +0530
From:   Vivek Gautam <vivek.gautam@...eaurora.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Will Deacon <will.deacon@....com>, Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org,
        open list <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH RFC 1/1] iommu/arm-smmu: Fix context fault message
 considering non-NTS

Hi Robin,


On Tue, Feb 6, 2018 at 5:22 PM, Robin Murphy <robin.murphy@....com> wrote:
> Hi Vivek,
>
> On 06/02/18 10:16, Vivek Gautam wrote:
>>
>> SMMU_CBn_FSYNR0 definition from SMMU v2 architecture document
>> says that, the S1CBNDX[23:16] field is only valid if SMMU_IDR0.NTS==1.
>> So, update the fsynr in context bank fault handler, so that the
>> fault message displays cbndx only when we have nested translations
>> enabled.
>> Otherwise, it is confusing when 'cb' gives the actual context
>> bank, while the 'fsynr' gives a different value.
>>
>> Example, on a sdm845 system:
>> Before this patch -
>> arm-smmu 15000000.apps-smmu: Unhandled context fault:
>>                            fsr=0x402, iova=0x9e0fb600, fsynr=0x3c0020,
>> cb=0
>> After this patch -
>> arm-smmu 15000000.apps-smmu: Unhandled context fault:
>>                            fsr=0x402, iova=0x9e0aa000, fsynr=0x20, cb=0
>>
>> Signed-off-by: Vivek Gautam <vivek.gautam@...eaurora.org>
>> Cc: Robin Murphy <robin.murphy@....com>
>> ---
>>
>> Hi Robin,
>> Does it make sense to mask fsynr like this?
>> Would it still be confusing for CB==0?
>> Tagging the patch as RFC to get your comments. If looks good,
>> then i will send the patch for your ack.
>> Thanks.
>
>
> I would expect that anyone who knows how to decode a raw FSYNR0 value should
> also be aware of when various fields are valid or not. I have page 302 of
> IHI0062D.c open next to this email, and to me it seems virtually impossible
> to read the definition of S1CBNDX without inherently seeing all the
> conditions under which it is UNKNOWN (which incidentally are more than this
> patch would cover).

Yea, I agree with you. To fully decode the S1CBNDX, one has to refer the spec,
and once one has the knowledge of the smmu implementation on the one's
target platform, it becomes unimportant to look at the cbndx again.

>
> In general, I don't agree with this - we're dumping the raw state of a
> register, and it's already a cryptic hex value that you have to refer to the
> architecture spec to decode. There is perhaps some argument for
> pretty-printing faults with everything fully decoded, but given that they
> should never be seen under normal operation (especially global faults), I
> really don't think that's worth the bother either.

Sure. We can drop this patch. Thanks.

>
> Robin.
>
>>   drivers/iommu/arm-smmu.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
>> index 69e7c60792a8..827659515b22 100644
>> --- a/drivers/iommu/arm-smmu.c
>> +++ b/drivers/iommu/arm-smmu.c
>> @@ -550,6 +550,9 @@ static irqreturn_t arm_smmu_context_fault(int irq,
>> void *dev)
>>         fsynr = readl_relaxed(cb_base + ARM_SMMU_CB_FSYNR0);
>>         iova = readq_relaxed(cb_base + ARM_SMMU_CB_FAR);
>>   +     if (!smmu->features & ARM_SMMU_FEAT_TRANS_NESTED)
>> +               fsynr &= 0xffff;
>> +
>>         dev_err_ratelimited(smmu->dev,
>>         "Unhandled context fault: fsr=0x%x, iova=0x%08lx, fsynr=0x%x,
>> cb=%d\n",
>>                             fsr, iova, fsynr, cfg->cbndx);
>>
> --
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