netdev - Re: How to debug DMAR errors?

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Date:   Fri, 14 Apr 2017 09:24:42 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Ben Greear <greearb@...delatech.com>
Cc:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: How to debug DMAR errors?

On Fri, Apr 14, 2017 at 9:19 AM, Ben Greear <greearb@...delatech.com> wrote:
>
>
> On 04/14/2017 08:45 AM, Alexander Duyck wrote:
>>
>> On Thu, Apr 13, 2017 at 11:12 AM, Ben Greear <greearb@...delatech.com>
>> wrote:
>>>
>>> Hello,
>>>
>>> I have been seeing a regular occurrence of DMAR errors, looking something
>>> like this when testing my ath10k driver/firmware under some specific
>>> loads
>>> (maximum receive of 512 byte frames in AP mode):
>>>
>>> DMAR: DRHD: handling fault status reg 3
>>> DMAR: [DMA Read] Request device [05:00.0] fault addr fd99f000 [fault
>>> reason
>>> 06] PTE Read access is not set
>>> ath10k_pci 0000:05:00.0: firmware crashed! (uuid
>>> 594b1393-ae35-42b5-9dec-74ff0c6791ff)
>>>
>>> So, I am wondering if there is any way I can get more information about
>>> what
>>> this fd99f000 address
>>> is?
>>>
>>> Once this problem hits, the entire OS locks hard (not even sysrq-boot
>>> will
>>> do anything),
>>> so I guess I would need the DMAR logic to print out more info on that
>>> address somehow.
>>>
>>> Thanks,
>>> Ben
>>
>>
>> There isn't much more info to give you. The problem is that the device
>> at 5:00.0 attempted to read at fd99f000 even though it didn't have
>> permissions. In response this should trigger a PCI Master Abort
>> message to that function. It looks like the firmware for the device
>> doesn't handle that and so that is likely why things got hung.
>>
>> Really you would need to interrogate the ath10k_pci to see if there
>> is/was a mapping somewhere for that address and what it was supposed
>> to be used for.
>
>
> I'm working on a hook in DMAR logic to call into ath10k_pci when the
> error is seen, so the ath10k can dump debug info, including recent DMA
> addresses.
>
> My code is an awful hack so far, but if someone could add a clean way to
> register
> DMAR error callbacks, I think that would be very welcome.  It might could
> tie into
> automated dma map/unmap debugging logic, and at the least, someone could
> write custom debugging callbacks
> for the driver(s) in question.
>
> Thanks,
> Ben
>

You might look at coding up something to add pci_error_handlers for
the pci_driver in the ath10k_pci driver. The PCI Master Abort should
trigger an error that you could then capture in the driver and handle
at least dumping it via your own implementation of the error handlers.
If nothing else I suspect there are probably some sort of descriptor
rings you could probably dump. I'm suspecting this is some sort of Tx
issue since the problem was a read fault, but I suppose there are
other paths in the driver that might trigger DMA read requests.

- Alex