Re: [PATCH V2 3/6] stm class: provision for statically assigned masterIDs

[Mathieu Poirier]

On 12 February 2016 at 09:27, Alexander Shishkin
<alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
> Mathieu Poirier <mathieu.poirier@xxxxxxxxxx> writes:
>
>> On 8 February 2016 at 06:26, Alexander Shishkin
>> <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
>>> This $end==$start situation itself may be ambiguous and can be
>>> interpreted either as having just one *static* master ID fixed for all
>>> SW writers (what I assumed from your commit message) or as having a
>>> floating master ID, which changes of its own accord and is not
>>> controllable by software.
>>
>> Some clarification here.
>>
>> On ARM from a SW point of view $end == $start and that doesn't change
>> (with regards to masterIDs) .  The ambiguity comes from the fact that
>> on other platforms where masterID configuration does change and is
>> important, the condition $end == $start could also be valid.
>
> Yes, that's what I was saying. The thing is, on the system-under-tracing
> side these two situations are not very different from one
> another. Master IDs are really just numbers without any semantics
> attached to them in the sense that they are not covered by the mipi spec
> or any other standard (to my knowledge).

We are definitely on the same page here, just using slightly different terms.

>
> The difference is in the way we map channels to masters. One way is to
> allocate a distinct set of channels for each master (the way Intel Trace
> Hub does it); another way is to share the same set of channels between
> multiple masters.

We are in total agreement.

> So we can describe this as "hardware implements the
> same set of channels across multiple masters" or something along those
> lines.

I suggest "Shared channels"?  In the end, that's really what it is...

The outstanding issue is still how to represent these to different way
of mapping things in the STM core.  I suggested a flag, called
"mstatic" (but that can be changed), and a representation of '-1' in
for masterIDs sysFS.  Whether we stick with that or not is irrelevant,
I'd be fine with another mechanism.  What I am keen on is that from
sysFS users can quickly tell which heuristic is enacted on that
specific architecture.

>
> Actually, in the latter scheme of things you can also have multiple
> masters, at least theoretically. Say, you have masters [0..15], each
> with distinct set of channels, but depending on hardware state these
> masters actually end up as $state*16+$masterID in the STP stream.
>
> So we might also think about differentiating between the hardware
> masters (0 though 15 in the above example) and STP masters.

I'm not sure I get what you mean here.  On ARM the masterIDs assigned
in HW, which will depend on the state, will show up in the STP stream.
But again, I might be missing your point.

Thanks,
Mathieu

>
> Regards,
> --
> Alex