linux-kernel - Re: [PATCH v5 10/15] coresight: trbe: Workaround TRBE errata overwrite in FILL mode

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Date:   Tue, 19 Oct 2021 09:37:36 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     will@...nel.org, catalin.marinas@....com, mike.leach@...aro.org,
        leo.yan@...aro.org, maz@...nel.org, coresight@...ts.linaro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 10/15] coresight: trbe: Workaround TRBE errata
 overwrite in FILL mode

On 19/10/2021 05:36, Anshuman Khandual wrote:
> 
> 
> On 10/19/21 2:45 AM, Suzuki K Poulose wrote:
>> On 18/10/2021 16:51, Mathieu Poirier wrote:
>>> On Thu, Oct 14, 2021 at 11:31:20PM +0100, Suzuki K Poulose wrote:
>>>> ARM Neoverse-N2 (#2139208) and Cortex-A710(##2119858) suffers from
>>>> an erratum, which when triggered, might cause the TRBE to overwrite
>>>> the trace data already collected in FILL mode, in the event of a WRAP.
>>>> i.e, the TRBE doesn't stop writing the data, instead wraps to the base
>>>> and could write upto 3 cache line size worth trace. Thus, this could
>>>> corrupt the trace at the "BASE" pointer.
>>>>
>>>> The workaround is to program the write pointer 256bytes from the
>>>> base, such that if the erratum is triggered, it doesn't overwrite
>>>> the trace data that was captured. This skipped region could be
>>>> padded with ignore packets at the end of the session, so that
>>>> the decoder sees a continuous buffer with some padding at the
>>>> beginning. The trace data written at the base is considered
>>>> lost as the limit could have been in the middle of the perf
>>>> ring buffer, and jumping to the "base" is not acceptable.
>>>> We set the flags already to indicate that some amount of trace
>>>> was lost during the FILL event IRQ. So this is fine.
>>>>
>>>> One important change with the work around is, we program the
>>>> TRBBASER_EL1 to current page where we are allowed to write.
>>>> Otherwise, it could overwrite a region that may be consumed
>>>> by the perf. Towards this, we always make sure that the
>>>> "handle->head" and thus the trbe_write is PAGE_SIZE aligned,
>>>> so that we can set the BASE to the PAGE base and move the
>>>> TRBPTR to the 256bytes offset.
>>>>
>>>> Cc: Mike Leach <mike.leach@...aro.org>
>>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>>>> Cc: Leo Yan <leo.yan@...aro.org>
>>>> Reviewed-by: Mathieu Poirier <mathieu.poirier@...aro.org>
>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>>> ---
>>>> Changes since v2:
>>>>    - Updated the ASCII art to include better description of
>>>>      all the steps in the work around
>>>> Change since v1:
>>>>    - Updated comment with ASCII art
>>>>    - Add _BYTES suffix for the space to skip for the work around.
>>>> ---
>>>>    drivers/hwtracing/coresight/coresight-trbe.c | 169 +++++++++++++++++--
>>>>    1 file changed, 158 insertions(+), 11 deletions(-)
>>>>
>>>> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
>>>> index 314e5e7374c7..b56b166b2dec 100644
>>>> --- a/drivers/hwtracing/coresight/coresight-trbe.c
>>>> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
>>>> @@ -16,6 +16,7 @@
>>>>    #define pr_fmt(fmt) DRVNAME ": " fmt
>>>>      #include <asm/barrier.h>
>>>> +#include <asm/cpufeature.h>
>>>
>>> Here too I get a checkpatch warning...
>>>
>>
>> That is a false alarm. I guess that warns for including
>> linux/cpufeature.h? It is a bit odd, we include this
>> for the arm64 cpucaps, not the generic linux feature
> 
> It is a bit odd, I saw that too.
> 
>> checks. (They are used for "loading modules" based
>> on "features" which are more like ELF HWCAPs).
> 
> Should <asm/cpufeature.h> be renamed as <asm/arm64_cpufeature.h>
> or something similar instead to differentiate it from the generic
> <linux/cpufeature.h> as they are not related. Also, probably this
> warning could be avoided.

It is not that simple. asm/cpufeature.h on arm64 provides :

  * arch backend for linux/cpufeature.h
  * CPU feature sanity check infrastructure
  * CPU capability infrastructure ( features & errata )

So ideally it should be split into 3 for better cleanup and
is something that could be pursued outside this series.

Suzuki