Re: [PATCH RFC 2/2] soc: Add a basic ACPI generic driver

[John Garry]

Hi Olof,

> > > Based on everything I've seen so far, this should go under drivers/acpi instead.
> >
> > soc drivers seem to live in drivers/soc (non-arm32, anyway), so I
> > decided on this location. But drivers/acpi would also seem reasonable now.
>
> We don't want drivers/soc to be too much of a catch-all -- it is meant
> for some of the glue pieces that don't have good homes elsewhere.
> Unfortunately, the slope is slippery and we've already gone down it a
> bit, but I think we can fairly clearly declare that this kind of
> cross-soc material is likely not the right home for it -- especially
> when drivers/acpi is a good fit in this case.

ok

> > > > diff --git a/drivers/soc/acpi_generic.c b/drivers/soc/acpi_generic.c
> > > > new file mode 100644
> > > > index 000000000000..34a1f5f8e063
> > > > --- /dev/null
> > > > +++ b/drivers/soc/acpi_generic.c
> > > > @@ -0,0 +1,102 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +/*
> > > > + * Copyright (c) John Garry, john.garry@xxxxxxxxxx
> > > > + */
> > > > +
> > > > +#define pr_fmt(fmt) "SOC ACPI GENERIC: " fmt
> > > > +
> > > > +#include <linux/acpi.h>
> > > > +#include <linux/sys_soc.h>
> > > > +

[...]

> > > Hmm, this doesn't look like much of a driver to me. This looks like
> > > the export of an attribute to userspace, and should probably be done
> > > by ACPI core instead of creating an empty driver for it.
> >
> > OK, but I'm thinking that having a soc driver can be useful as it is
> > common to DT, and so userspace only has to check a single location. And
> > the soc driver can also cover multiple-chip systems without have to
> > reinvent that code for ACPI core. And it saves adding a new ABI.
>
> While having a single location could be convenient, the actual data
> read/written would be different (I'm guessing).

Without doubt we would have different data sometimes between ACPI and DT 
FW..

And it is not ideal that the soc_id sysfs file could have different 
contents for the same SoC, depending on ACPI or DT.

> We also already have a supposed standard way of figuring out what SoC
> we're on (toplevel compatible for the DT). 

From checking some soc drivers, there is a distinction between how 
soc_id and machine is evaluated: machine comes from DT model, which 
looks standard; however soc_id seems to have different methods of 
evaluate, like sometimes reading some system id register (I'm checking 
exynos-chipid.c there).

We're just looking for soc_id. But, as before, it would probably be 
different between ACPI and DT, so not ideal.

So no matter what, I think
> userspace will need to handle two ways of probing this.

That should not be a big problem.

>

Thanks,
John