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

[Suzuki K Poulose]

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