Re: [PATCH] coresight: etm4x: request to retain power to the trace unit when active

[Sudeep Holla]

On 22/06/16 11:46, Suzuki K Poulose wrote:
> On 22/06/16 11:15, Sudeep Holla wrote:
> > The Coresight ETMv4 architecture provides a way to request to keep the
> > power to the trace unit. This might help to collect the traces without
> > the need to disable the CPU power management(entering/exiting deeper
> > idle states).
> >
> > Trace PowerDown Control Register provides powerup request bit which when
> > set requests the system to retain power to the trace unit and emulate
> > the powerdown request.
> >
> > Typically, a trace unit drives a signal to the power controller to
> > request that the trace unit core power domain is powered up. However,
> > if the trace unit and the CPU are in the same power domain then the
> > implementation might combine the trace unit power up status with a
> > signal from the CPU.
> >
> > This patch requests to retain power to the trace unit when active and
> > to remove when inactive. Note this change will only request but the
> > behaviour depends on the implementation. However, it matches the
> > exact behaviour expected when the external debugger is connected with
> > respect to CPU power states.
> >
> > Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> > Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
>
>
> Thanks for debugging this issue patiently and fixing it :)
> One comment below.

[...]

> > @@ -293,6 +299,9 @@ static void etm4_disable_hw(void *info)
>
> > +    /* power can be removed from the trace unit now */
> > +    writel_relaxed(0, drvdata->base + TRCPDCR);
> > +
>
> At the moment the other bits in TRCPDCR are reserved (RES0). However
> to prevent issues in the future, it would be safer to read the value
> and set/clear the Bit of our interest than blindly writing those values.

Agreed, I had done exactly like than when I wrote it initially and then
change seeing few other writes in the driver. Anyways fixed now and v2
on its way.

--
Regards,
Sudeep