linux-kernel - Re: [PATCH v2] coresight: tmc-etr: Speed up for bounce buffer in flat mode

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Message-ID: <20210712113646.GD704210@leoy-ThinkPad-X240s>
Date:   Mon, 12 Jul 2021 19:36:46 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] coresight: tmc-etr: Speed up for bounce buffer in
 flat mode

On Mon, Jul 12, 2021 at 12:17:04PM +0100, Suzuki Kuruppassery Poulose wrote:

[...]

> > I am doubt why you conclude "always start tracing at the beginning of
> > the buffer"?  I read the driver but cannot find any code in the driver
> > to reset rrp and rwp after fetching the trace data, or there have any
> > implict operation to reset pointers?
> 
> The ETR is always programmed with the base address of the "ETR" buffer,
> which is *not the same* as the perf ring buffer, since we always do
> double buffering. We do not program the RRP/RWP of the ETR (except
> for the SoC-600, where it is mandatory and we set them to the base
> address). Thus there is no context associated with the ETR buffer.
> But at the end of the run, we do read the RRP/ RWP to figure out
> where the ETR has reached.
> 
> As for reseting the RRP / RWP, at the beginning of a session, is
> done implicitly for the ETR (except for SoC-600 ETRs as explained
> above) by the hardware to the base address.

Yes, I finally matched your description with the code.  Will respin
patch for this.

Thanks for confirmation!
Leo