linux-kernel - Re: [PATCH V4 0/6] coresight: etm4x: Migrate ACPI AMBA devices to platform driver

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Date:   Tue, 23 May 2023 16:51:59 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, suzuki.poulose@....com,
        Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
        Steve Clevenger <scclevenger@...amperecomputing.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>, devicetree@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH V4 0/6] coresight: etm4x: Migrate ACPI AMBA devices to
 platform driver

On Tue, May 23, 2023 at 10:15:47AM +0530, 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 6.4-rc3.
> 

Tested on Juno with some hacked up UEFI f/w.

Tested-by: Sudeep Holla <sudeep.holla@....com>

-- 
Regards,
Sudeep