[PATCH 0/7] coresight: etm4x: Migrate AMBA devices to platform driver

Anshuman Khandual
2023-03-17T03:05:26Z

CoreSight ETM4x devices could be accessed either via MMIO (handled via
amba_driver) or CPU system instructions (handled via platform driver). But
this has the following issues :

- Each new CPU comes up with its own PID and thus we need to keep on
adding the "known" PIDs to get it working with AMBA driver. While
the ETM4 architecture (and CoreSight architecture) defines way to
identify a device as ETM4. Thus older kernels won't be able to
"discover" a newer CPU, unless we add the PIDs.

- With ACPI, the ETM4x devices have the same HID to identify the device
irrespective of the mode of access. This creates a problem where two
different drivers (both AMBA based driver and platform driver) would
hook into the "HID" and could conflict. e.g., if AMBA driver gets
hold of a non-MMIO device, the probe fails. If we have single driver
hooked into the given "HID", we could handle them seamlessly,
irrespective of the mode of access.

- CoreSight is heavily dependent on the runtime power management. With
ACPI, amba_driver doesn't get us anywhere with handling the power
and thus one need to always turn the power ON to use them. Moving to
platform driver gives us the power management for free.

Due to all of the above, we are moving the MMIO based etm4x devices to be
supported via platform driver. The series makes the existing platform
driver generic to handle both type of the access modes. With that we can
also remove the etm4x amba driver.

Finally, we need a way to make sure the new driver gets control of the
ETM4x device on a DT based system. CoreSight devices have always had the
"arm,primecell" in the compatible list. But the way this is handled
currently in OF code is a bit messy. The ETM4x devices are identified by
"arm,coresight-etm4x". The platform driver can never get a chance to probe
these devices, since the "arm,primecell" takes priority and is hard-coded
in the OF code. We have two options here :

1) Remove the arm,primecell from all DTS. This is fine for "new" kernels
with this change. But, for existing boards, using an older kernel will
break. Thus, is not preferred.

2) Add a white list of "compatibles" where the "priority" of the
"arm,primecell" can be ignored.

The series implements (2) above and applies on 6.3-rc2.

Cc: Steve Clevenger <scclevenger@os.amperecomputing.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Russell King (Oracle) <linux@armlinux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: devicetree@vger.kernel.org
Cc: linux-acpi@vger.kernel.org
Cc: coresight@lists.linaro.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Anshuman Khandual (6):
coresight: etm4x: Allocate and device assign 'struct etmv4_drvdata' earlier
coresight: etm4x: Drop iomem 'base' argument from etm4_probe()
coresight: etm4x: Drop pid argument from etm4_probe()
coresight: etm4x: Change etm4_platform_driver driver for MMIO devices
of/platform: Skip coresight etm4x devices from AMBA bus
coresight: etm4x: Drop the AMBA driver

Suzuki Poulose (1):
coresight: etm4x: Add ACPI support in platform driver

drivers/acpi/acpi_amba.c | 1 -
.../coresight/coresight-etm4x-core.c | 171 ++++++++----------
drivers/hwtracing/coresight/coresight-etm4x.h | 3 +
drivers/of/platform.c | 10 +-
include/linux/coresight.h | 56 ++++++
5 files changed, 143 insertions(+), 98 deletions(-)

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2.25.1