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

Al Grant [off-list ref] writes:

quoted

Mike did write "master IDs are hardwired to individual cores and core security
states", which make assignment for one platform very static.
On the flip side those will change from one system to another.

It depends on your perspective.  From the perspective of a userspace
process not pinned to a core, the master id will appear to vary
dynamically and unpredictably as the thread migrates from one
core to another.  (That's actually useful if the decoder wants to know
where the thread is running at any given point, as it can find that out
for free, without the need to track migration events.)

On the other hand if you are pinned (e.g. you're the kernel on a
particular core, or you're a per-core worker thread in some thread
pooling system) then you have a fixed master id, and then you can
have one instance per core all using the same range of channel
numbers, with the master id indicating the core - this saves on
channel space compared to having to give each core its own
range of channel space.

What I understood from Mike's explanation is that basically nothing is
fixed from the software perspective (talking about Coresight STM). One
doesn't know the master id with which one's data will be tagged,
regardless of whether one is cpu-affine or not, because:

 * even per-core master IDs will be highly implementation dependent and
   therefore not knowable by the kernel and subsequently userspace,
 * master IDs may change based on other conditions (besides security
   states), so even if we knew per-core master IDs for sure, it wouldn't
   give us the complete picture.

So even though from the HW perspective master IDs may be "hardwired" to
hardware states, the hardware states themselves are dynamic, so from the
SW perspective these master IDs are not static or hardwired at all.

If the same mmio write done by software would always result in the exact
same master ID tagged to it, that would be more like "static" to me.

Regards,
--
Alex