linux-kernel - Re: [PATCH 1/4] amba: Export amba

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Date:   Wed, 9 May 2018 09:40:55 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Kim Phillips <kim.phillips@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Gargi Sharma <gs051095@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Rik van Riel <riel@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Thierry Reding <treding@...dia.com>,
        Todd Kjos <tkjos@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] amba: Export amba_bustype

On Wed, May 09, 2018 at 02:38:32PM +0100, Robin Murphy wrote:
> Hi Kim,
> 
> On 08/05/18 20:06, Kim Phillips wrote:
> >This patch is provided in the context of allowing the Coresight driver
> >subsystem to be loaded as modules.  Coresight uses amba_bus in its call
> >to bus_find_device() in of_coresight_get_endpoint_device() when
> >searching for a configurable endpoint device.  This patch allows
> >Coresight to reference amba_bustype when built as a module.
> >
> >Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> >Cc: Alex Williamson <alex.williamson@...hat.com>
> >Cc: Eric Auger <eric.auger@...hat.com>
> >Cc: Russell King <linux@...linux.org.uk>
> >Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >Cc: Todd Kjos <tkjos@...gle.com>
> >Cc: Geert Uytterhoeven <geert+renesas@...der.be>
> >Cc: Thierry Reding <treding@...dia.com>
> >Cc: Robin Murphy <robin.murphy@....com>
> >Signed-off-by: Kim Phillips <kim.phillips@....com>
> >---
> >There was a prior patch submitted by Alex W. here:
> >
> >https://lkml.org/lkml/2017/6/19/811
> >
> >But I can't tell its fate - presume simply delayed?
> >
> >Coresight uses amba_bus in its call to bus_find_device() here:
> >
> >https://lxr.missinglinkelectronics.com/linux/drivers/hwtracing/coresight/of_coresight.c#L51
> >
> >Grepping for bus_type and EXPORT shows other busses exporting their
> >type, so I don't think this is the wrong approach.  If, OTOH, Coresight
> >needs to do something differently, please comment.
> 
> Exposing raw bus_types is pretty ugly, but it is indeed the status quo, so
> this probably is the reasonable thing to do. I suppose an amba_bus
> equivalent of of_find_device_by_node() could be implemented, but for only a
> single potential user that doesn't seem particularly worthwhile, since
> unless some massive shake-up of how buses work comes along the bus_type will
> inevitably end up being exported for other reasons anyway. So, in the
> context of this series;
> 
> Reviewed-by: Robin Murphy <robin.murphy@....com>
> 
> However, as a wild idea for sidestepping the issue completely (or at least
> keeping it within the CoreSight framework), at first glance it appears
> something like the below might be feasible, although I may well be missing
> some obvious reason why not.
> 
> Thanks,
> Robin.
> 
> ----->8-----
> diff --git a/drivers/hwtracing/coresight/of_coresight.c
> b/drivers/hwtracing/coresight/of_coresight.c
> index 7c375443ede6..2c3fdc9b63e6 100644
> --- a/drivers/hwtracing/coresight/of_coresight.c
> +++ b/drivers/hwtracing/coresight/of_coresight.c
> @@ -27,28 +27,13 @@
> 
>  static int of_dev_node_match(struct device *dev, void *data)
>  {
> -	return dev->of_node == data;
> +	return dev->parent->of_node == data;
>  }
> 
>  static struct device *
>  of_coresight_get_endpoint_device(struct device_node *endpoint)
>  {
> -	struct device *dev = NULL;
> -
> -	/*
> -	 * If we have a non-configurable replicator, it will be found on the
> -	 * platform bus.
> -	 */
> -	dev = bus_find_device(&platform_bus_type, NULL,
> -			      endpoint, of_dev_node_match);
> -	if (dev)
> -		return dev;
> -
> -	/*
> -	 * We have a configurable component - circle through the AMBA bus
> -	 * looking for the device that matches the endpoint node.
> -	 */
> -	return bus_find_device(&amba_bustype, NULL,
> +	return bus_find_device(&coresight_bustype, NULL,
>  			       endpoint, of_dev_node_match);
>  }

Hi Robin and thanks for the input.  

Your approach would work if all CS devices would be on the CS bus, which is not
the case at discovery time when of_coresight_get_endpoint_device() is called.

Mathieu

>