On 17/03/2023 20:06, Rob Herring wrote:
On Fri, Mar 17, 2023 at 11:03 AM Suzuki K Poulose
<suzuki.poulose@xxxxxxx> wrote:
Hi Rob
Thanks for your response.
On 17/03/2023 14:52, Rob Herring wrote:
On Thu, Mar 16, 2023 at 10:06 PM Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:
Allow other drivers to claim a device, disregarding the "priority" of
"arm,primecell". e.g., CoreSight ETM4x devices could be accessed via MMIO
(AMBA Bus) or via CPU system instructions.
The OS can pick which one, use both, or this is a system integration
time decision?
Not an OS choice. Historically, this has always been MMIO accessed but
with v8.4 TraceFiltering support, CPUs are encouraged to use system
instructions and obsolete MMIO. So, yes, MMIO is still possible but
something that is discouraged and have to be decided at system
integration time.
The CoreSight ETM4x platform
driver can now handle both types of devices. In order to make sure the
driver gets to handle the "MMIO based" devices, which always had the
"arm,primecell" compatible, we have two options :
1) Remove the "arm,primecell" from the DTS. But this may be problematic
for an older kernel without the support.
2) The other option is to allow OF code to "ignore" the arm,primecell
priority for a selected list of compatibles. This would make sure that
both older kernels and the new kernels work fine without breaking
the functionality. The new DTS could always have the "arm,primecell"
removed.
3) Drop patches 6 and 7 and just register as both AMBA and platform
drivers. It's just some extra boilerplate. I would also do different
compatible strings for CPU system instruction version (assuming this
is an integration time decision).
The system instruction (and the reigster layouts) are all part of the
ETMv4/ETE architecture and specific capabilities/features are
discoverable, just like the Arm CPUs. Thus we don't need special
versions within the ETMv4x or ETE minor versions. As of now, we have
one for etm4x and another for ete.
I just meant 2 new compatible strings. One each for ETMv4x and ETE,
but different from the 2 existing ones. It is different h/w presented
to the OS, so different compatible.
Sorry, was not very clear here.
Right now, we have :
1) arm,coresight-etm4x && arm,primecell - For AMBA based devices
2) arm,coresight-etm4x-sysreg - For system instruction based
3) arm,embedded-trace-extension - For ETE
One problem with the AMBA driver in place is having to keep on adding
new PIDs for the CPUs. The other option is to have a blanket mask
for matching the PIDs with AMBA_UCI_ID checks.
But if MMIO access is discouraged, then new h/w would use the platform
driver(s), not the amba driver, and you won't have to add PIDs.
Yes for v8.4 onwards. Alternatively, the newer DTS could skip
arm,primecell in the entry and that would kick the platform driver
in. So, that should be fine I guess.
Kind regards
Suzuki
Rob
