Re: [PATCH 6/7] of/platform: Skip coresight etm4x devices from AMBA bus

[Rob Herring <robh+dt@kernel.org>]

On Thu, Mar 16, 2023 at 10:06 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> Allow other drivers to claim a device, disregarding the "priority" of
> "arm,primecell". e.g., CoreSight ETM4x devices could be accessed via MMIO
> (AMBA Bus) or via CPU system instructions.

The OS can pick which one, use both, or this is a system integration
time decision?

> The CoreSight ETM4x platform
> driver can now handle both types of devices. In order to make sure the
> driver gets to handle the "MMIO based" devices, which always had the
> "arm,primecell" compatible, we have two options :
>
> 1) Remove the "arm,primecell" from the DTS. But this may be problematic
>  for an older kernel without the support.
>
> 2) The other option is to allow OF code to "ignore" the arm,primecell
> priority for a selected list of compatibles. This would make sure that
> both older kernels and the new kernels work fine without breaking
> the functionality. The new DTS could always have the "arm,primecell"
> removed.

3) Drop patches 6 and 7 and just register as both AMBA and platform
drivers. It's just some extra boilerplate. I would also do different
compatible strings for CPU system instruction version (assuming this
is an integration time decision).

>
> This patch implements Option (2).
>
> 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: devicetree@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Co-developed-by: Suzuki Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Suzuki Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  drivers/of/platform.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index b2bd2e783445..59ff1a38ccaa 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -325,6 +325,13 @@ static const struct of_dev_auxdata *of_dev_lookup(const struct of_dev_auxdata *l
>         return NULL;
>  }
>
> +static const struct of_device_id of_ignore_amba_table[] = {
> +#ifdef CONFIG_CORESIGHT_SOURCE_ETM4X
> +       { .compatible = "arm,coresight-etm4x" },
> +#endif
> +       {}    /* NULL terminated */
> +};
> +
>  /**
>   * of_platform_bus_create() - Create a device for a node and its children.
>   * @bus: device node of the bus to instantiate
> @@ -373,7 +380,8 @@ static int of_platform_bus_create(struct device_node *bus,
>                 platform_data = auxdata->platform_data;
>         }
>
> -       if (of_device_is_compatible(bus, "arm,primecell")) {
> +       if (of_device_is_compatible(bus, "arm,primecell") &&
> +           unlikely(!of_match_node(of_ignore_amba_table, bus))) {

of_match_node is going to take orders of magnitude longer than any
difference unlikely() would make. Drop it.

>                 /*
>                  * Don't return an error here to keep compatibility with older
>                  * device tree files.
> --
> 2.25.1
>