Re: [RFC v5 net-next 08/13] mfd: add interface to check whether a device is mfd

[Colin Foster <colin.foster@xxxxxxxxxxxxxxxx>]

On Wed, Dec 29, 2021 at 03:25:55PM +0000, Lee Jones wrote:
> On Sat, 18 Dec 2021, Colin Foster wrote:
> 
> > Some drivers will need to create regmaps differently based on whether they
> > are a child of an MFD or a standalone device. An example of this would be
> > if a regmap were directly memory-mapped or an external bus. In the
> > memory-mapped case a call to devm_regmap_init_mmio would return the correct
> > regmap. In the case of an MFD, the regmap would need to be requested from
> > the parent device.
> > 
> > This addition allows the driver to correctly reason about these scenarios.
> > 
> > Signed-off-by: Colin Foster <colin.foster@xxxxxxxxxxxxxxxx>
> > ---
> >  drivers/mfd/mfd-core.c   |  5 +++++
> >  include/linux/mfd/core.h | 10 ++++++++++
> >  2 files changed, 15 insertions(+)
> > 
> > diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
> > index 684a011a6396..905f508a31b4 100644
> > --- a/drivers/mfd/mfd-core.c
> > +++ b/drivers/mfd/mfd-core.c
> > @@ -33,6 +33,11 @@ static struct device_type mfd_dev_type = {
> >  	.name	= "mfd_device",
> >  };
> >  
> > +int device_is_mfd(struct platform_device *pdev)
> > +{
> > +	return (!strcmp(pdev->dev.type->name, mfd_dev_type.name));
> > +}
> > +
> 
> Why is this device different to any other that has ever been
> mainlined?

Hi Lee,

First, let me apologize for not responding to your response from the
related RFC from earlier this month. It had been blocked by my spam
filter and I had not seen it until just now. I'll have to check that
more diligently now.

Moving on...

That's a question I keep asking myself. Either there's something I'm
missing, or there's something new I'm doing.

This is taking existing drivers that work via MMIO regmaps and making
them interface-independent. As Vladimir pointed out here:
https://lore.kernel.org/all/20211204022037.dkipkk42qet4u7go@skbuf/T/
device_is_mfd could be dropped in lieu of an mfd-specific probe
function.

If there's something I'm missing, please let me know. But it feels like
devm_get_regmap_from_resource at the end of the day would be the best
solution to the design, and that doesn't exist. And implementing
something like that is a task that I feel I'm not capable of tackling at
this time.

> 
> -- 
> Lee Jones [李琼斯]
> Senior Technical Lead - Developer Services
> Linaro.org │ Open source software for Arm SoCs
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