linux-kernel - Re: [PATCH 10/11] coresight-etm4x: Controls pertaining to the context ID functions

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Date:	Thu, 23 Apr 2015 12:59:51 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Mathieu Poirier <mathieu.poirier@...aro.org>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-api@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kaixu Xia <kaixu.xia@...aro.org>,
	Chunyan Zhang <zhang.chunyan@...aro.org>
Subject: Re: [PATCH 10/11] coresight-etm4x: Controls pertaining to the
 context ID functions

On 04/23/2015 11:16 AM, Mathieu Poirier wrote:
> On 23 April 2015 at 09:08, Christopher Covington <cov@...eaurora.org> wrote:
>> Hi Mathieu,
>>
>> On 04/22/2015 06:40 PM, Mathieu Poirier wrote:
>>> From: Pratik Patel <pratikp@...eaurora.org>
>>>
>>> Adding sysfs entries to access and configure specifics about the
>>> context ID comparator functions.
>>>
>>> Signed-off-by: Pratik Patel <pratikp@...eaurora.org>
>>> Signed-off-by: Mathieu Poirier <mathieu.poirier@...aro.org>
>>> ---
>>>  .../ABI/testing/sysfs-bus-coresight-devices-etm4x  |  19 +++
>>>  drivers/hwtracing/coresight/coresight-etm4x.c      | 187 +++++++++++++++++++++
>>>  2 files changed, 206 insertions(+)
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-bus-coresight-devices-etm4x b/Documentation/ABI/testing/sysfs-bus-coresight-devices-etm4x
>>> index b5c0456290ab..8b32cb7b9723 100644
>>> --- a/Documentation/ABI/testing/sysfs-bus-coresight-devices-etm4x
>>> +++ b/Documentation/ABI/testing/sysfs-bus-coresight-devices-etm4x
>>> @@ -254,3 +254,22 @@ Date:            April 2015
>>>  KernelVersion:       4.01
>>>  Contact:     Mathieu Poirier <mathieu.poirier@...aro.org>
>>>  Description:         (RW) Controls the selection of the resources in the trace unit.
>>> +
>>> +What:                /sys/bus/coresight/devices/<memory_map>.etm/ctxid_idx
>>> +Date:                April 2015
>>> +KernelVersion:       4.01
>>> +Contact:     Mathieu Poirier <mathieu.poirier@...aro.org>
>>> +Description: (RW) Select which context ID comparator to work with.
>>> +
>>> +What:                /sys/bus/coresight/devices/<memory_map>.etm/ctxid_val
>>> +Date:                April 2015
>>> +KernelVersion:       4.01
>>> +Contact:     Mathieu Poirier <mathieu.poirier@...aro.org>
>>> +Description: (RW) Get/Set the context ID comparator value to trigger on.
>>
>> If I'm understanding this correctly, ctxid_val is multiplexed using ctxid_idx.
>> Why not just have ctxid_val0, ctxid_val1, ...?
> 
> There can be up to 8 of them on the current implementation and who
> knows how many more in the future.  To me using and index to select
> the context ID to work on scaled better and is introducing less
> clutter in sysfs.

In theory there's a race condition if two tasks are trying to set the index
and then the value, which having individually addressable values solves, but I
don't anticipate trying to use more than one task to configure tracing.

Chris

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