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

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Date:   Wed, 29 Jan 2020 09:58:04 +0000
From:   John Garry <john.garry@...wei.com>
To:     Olof Johansson <olof@...om.net>
CC:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "jeremy.linton@....com" <jeremy.linton@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "Guohanjun (Hanjun Guo)" <guohanjun@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH RFC 2/2] soc: Add a basic ACPI generic driver

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@...wei.com
>>>> + */
>>>> +
>>>> +#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