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

[Rob Herring]

On Tue, May 8, 2018 at 10:48 AM, Suzuki K Poulose
<Suzuki.Poulose at arm.com> wrote:
> On 04/05/18 23:56, Rob Herring wrote:
>>
>> On Thu, May 3, 2018 at 3:32 PM, Mathieu Poirier
>> <mathieu.poirier at linaro.org> wrote:
>>>
>>> On 1 May 2018 at 07:13, Rob Herring <robh at kernel.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.
>
>
>
>>>>> ---
>>>>>   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.
>>
>>
>> No. If we had a bunch of cases, then we'd probably want to have just
>> 'scatter-gather'.
>
>
> Does it mean "arm,scatter-gather" ?

Yes. Use that.

> If we ever wanted to add the device
> specific information, I would prefer to go with "arm,tmc-scatter-gather"
> and not "etr-scatter-gather". They both could mean different things.
>
>>
>> BTW, if SG had already been supported, then I'd say this is a quirk
>> and we should invert this property. Otherwise, you'd be disabling once
>> enabled SG and require working platforms to update their dtb. Of
>> course, I shouldn't really let the state of an OS driver influence the
>> DT binding.
>>
>
> The SG support is added with this series. So, the OS has never made use
> of the feature.

Linux never did, but other OSs use DT, hence why I said "an OS
driver", not "the OS driver". But in reality, I'd guess only Linux has
Coresight support at all.

Rob