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

[Suzuki K Poulose]

On 04/05/18 23:56, Rob Herring wrote:
> On Thu, May 3, 2018 at 3:32 PM, Mathieu Poirier
> <mathieu.poirier@xxxxxxxxxx> wrote:
> > On 1 May 2018 at 07:13, Rob Herring <robh@xxxxxxxxxx> 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" ? 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.

Suzuki