Re: [PATCH V6 0/6] coresight: etm4x: Migrate ACPI AMBA devices to platform driver

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On 10/07/2023 07:24, Anshuman Khandual wrote:
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 ACPI MMIO based etm4x devices to be
supported via tha platform driver. The series makes the existing platform
driver generic to handle both type of the access modes. Although existing
AMBA driver would still continue to support DT based etm4x MMIO devices.
Although some problems still remain, such as manually adding PIDs for all
new AMBA DT based devices.

The series applies on 6.5-rc1.

Changes in V6:

- Rebased on 6.5-rc1


I have queued this version for v6.6, should appear on coresight/next soon.

Suzuki





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