linux-kernel - Re: [PATCH v2 10/27] dts: bindings: Restrict coresight tmc-etr scatter-gather mode

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Date:   Thu, 3 May 2018 14:32:40 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel@...r.kernel.org, Mike Leach <mike.leach@...aro.org>,
        Robert Walker <robert.walker@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Robin Murphy <robin.murphy@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Frank Rowand <frowand.list@...il.com>,
        John Horley <john.horley@....com>,
        Mathieu Poirier <matheiu.poirier@...aro.org>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v2 10/27] dts: bindings: Restrict coresight tmc-etr
 scatter-gather mode

On 1 May 2018 at 07:13, Rob Herring <robh@...nel.org> wrote:
> On Tue, May 01, 2018 at 10:10:40AM +0100, Suzuki K Poulose wrote:
>> We are about to add the support for ETR builtin scatter-gather mode
>> for dealing with large amount of trace buffers. However, on some of
>> the platforms, using the ETR SG mode can lock up the system due to
>> the way the ETR is connected to the memory subsystem.
>>
>> In SG mode, the ETR performs READ from the scatter-gather table to
>> fetch the next page and regular WRITE of trace data. If the READ
>> operation doesn't complete(due to the memory subsystem issues,
>> which we have seen on a couple of platforms) the trace WRITE
>> cannot proceed leading to issues. So, we by default do not
>> use the SG mode, unless it is known to be safe on the platform.
>> We define a DT property for the TMC node to specify whether we
>> have a proper SG mode.
>>
>> Cc: Mathieu Poirier <matheiu.poirier@...aro.org>
>> Cc: Mike Leach <mike.leach@...aro.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: John Horley <john.horley@....com>
>> Cc: Robert Walker <robert.walker@....com>
>> Cc: devicetree@...r.kernel.org
>> Cc: frowand.list@...il.com
>> Cc: Rob Herring <robh@...nel.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>  Documentation/devicetree/bindings/arm/coresight.txt | 3 +++
>>  drivers/hwtracing/coresight/coresight-tmc.c         | 8 +++++++-
>>  2 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/devicetree/bindings/arm/coresight.txt b/Documentation/devicetree/bindings/arm/coresight.txt
>> index cdd84d0..7c0c8f0 100644
>> --- a/Documentation/devicetree/bindings/arm/coresight.txt
>> +++ b/Documentation/devicetree/bindings/arm/coresight.txt
>> @@ -88,6 +88,9 @@ its hardware characteristcs.
>>       * arm,buffer-size: size of contiguous buffer space for TMC ETR
>>        (embedded trace router)
>>
>> +     * scatter-gather: boolean. Indicates that the TMC-ETR can safely
>> +       use the SG mode on this system.
>> +
>
> Needs a vendor prefix.
>

Thinking further on this, do we need to make it device specific as
well - something like "arm,etr-scatter-gather"?  That way we don't
have to redefine "scatter-gather" for other ARM devices if they happen
to need the same property but for different reasons.

>>  * Optional property for CATU :
>>       * interrupts : Exactly one SPI may be listed for reporting the address
>>         error