Re: [PATCH 1/4] amba: Export amba_bustype

[Robin Murphy]

On 09/05/18 16:40, Mathieu Poirier wrote:
> 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@xxxxxxxxxx>
> > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > Cc: Eric Auger <eric.auger@xxxxxxxxxx>
> > > Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Todd Kjos <tkjos@xxxxxxxxxx>
> > > Cc: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> > > Cc: Thierry Reding <treding@xxxxxxxxxx>
> > > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > > Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
> > > ---
> > > 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@xxxxxxx>
> >
> > 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.

Ah, now I see that obvious thing I was indeed missing - I got mixed up 
and started thinking bus_add_device() only got called as part of driver 
probe, but of course it's actually much earlier in 
{platform,amba}_device_add(). That means my idea cannot in fact work at 
all, since both ends of any link would defer waiting for the other to 
probe (and thus call coresight_register()) first. Oh well, I guess 
poking amba_bustype really does remain the only reasonable answer.

Thanks,
Robin.