linux-kernel - Re: [PATCH 0/7] coresight: etm4x: Migrate AMBA devices to platform driver

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Message-ID: <CAL_JsqKsnq0d-x3m3xQe8m0pnk_Jeh9J1oFBtPAn3LV8-MFH0w@mail.gmail.com>
Date:   Mon, 20 Mar 2023 09:17:16 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org,
        suzuki.poulose@....com, scclevenger@...amperecomputing.com,
        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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] coresight: etm4x: Migrate AMBA devices to platform driver

On Thu, Mar 16, 2023 at 10:05 PM Anshuman Khandual
<anshuman.khandual@....com> 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.

But v8.4 discourages MMIO access, so this problem will go away on its
own. Even if not, adding IDs to stable kernels is standard practice
whether it is PCI VID/PID, compatible string or AMBA PID.

>   - 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.

Why are we changing DT for ACPI? Just always use the platform driver
for ACPI and leave DT systems alone.

>   - 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.

This sounds like an issue for any amba driver. If this is an issue,
solve it for everyone, not just work around it in one driver.

When someone puts another primecell device into an ACPI system, are we
going to go do the same one-off change in that driver too? (We kind of
already did with SBSA UART...)

Rob