linux-kernel - Re: [PATCH 2/2] mfd: mfd-core: Honour Device Tree's request to disable a child-device

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Message-ID: <20191019072809.GX4365@dell>
Date:   Sat, 19 Oct 2019 08:28:09 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     broonie@...nel.org, linus.walleij@...aro.org,
        daniel.thompson@...aro.org, arnd@...db.de, baohua@...nel.org,
        stephan@...hold.net, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] mfd: mfd-core: Honour Device Tree's request to
 disable a child-device

Good morning Robin,

It's been a while.  I hope that you are well.

Thanks for taking an interest.

On Fri, 18 Oct 2019, Robin Murphy wrote:
> On 18/10/2019 13:26, Lee Jones wrote:
> > Until now, MFD has assumed all child devices passed to it (via
> > mfd_cells) are to be registered.  It does not take into account
> > requests from Device Tree and the like to disable child devices
> > on a per-platform basis.
> > 
> > Well now it does.
> > 
> > Reported-by: Barry Song <Baohua.Song@....com>
> > Reported-by: Stephan Gerhold <stephan@...hold.net>
> > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > ---
> >   drivers/mfd/mfd-core.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
> > index eafdadd58e8b..24c139633524 100644
> > --- a/drivers/mfd/mfd-core.c
> > +++ b/drivers/mfd/mfd-core.c
> > @@ -182,6 +182,11 @@ static int mfd_add_device(struct device *parent, int id,
> >   	if (parent->of_node && cell->of_compatible) {
> >   		for_each_child_of_node(parent->of_node, np) {
> >   			if (of_device_is_compatible(np, cell->of_compatible)) {
> > +				if (!of_device_is_available(np)) {
> > +					/* Ignore disabled devices error free */
> > +					ret = 0;
> > +					goto fail_alias;
> > +				}
> 
> Is it possible for a device to have multiple children of the same type? If
> so, it seems like this might not work as desired if, say, the first child
> was disabled but subsequent ones weren't.
> 
> It might make sense to use for_each_available_child_of_node() for the outer
> loop, then check afterwards if anything was found.

The subsystem in its current guise doesn't reliably support the
situation you describe. We have no way to track which child nodes have
been through this process previously, thus mfd-core will always choose
the first child node with a matching compatible string.

If you have any suggests in terms of adding support for multiple
children with matching compatible strings, I'd be very receptive.

> >   				pdev->dev.of_node = np;
> >   				pdev->dev.fwnode = &np->fwnode;
> >   				break;
> > 

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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