linux-kernel - Re: [PATCH v2 2/6] ACPI: bus: update acpi_dev_uid

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Date:   Wed, 22 Nov 2023 12:55:39 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Raag Jadav <raag.jadav@...el.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        mika.westerberg@...ux.intel.com, andriy.shevchenko@...ux.intel.com,
        lenb@...nel.org, robert.moore@...el.com, ardb@...nel.org,
        will@...nel.org, mark.rutland@....com, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        acpica-devel@...ts.linuxfoundation.org, linux-efi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        mallikarjunappa.sangannavar@...el.com, bala.senthil@...el.com
Subject: Re: [PATCH v2 2/6] ACPI: bus: update acpi_dev_uid_match() to support
 multiple types

On Wed, Nov 22, 2023 at 5:58 AM Raag Jadav <raag.jadav@...el.com> wrote:
>
> On Tue, Nov 21, 2023 at 08:25:20PM +0100, Rafael J. Wysocki wrote:
> > On Tue, Nov 21, 2023 at 11:38 AM Raag Jadav <raag.jadav@...el.com> wrote:
> > >
> > > According to ACPI specification, a _UID object can evaluate to either
> > > a numeric value or a string. Update acpi_dev_uid_match() helper to
> > > support _UID matching for both integer and string types.
> > >
> > > Suggested-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > > Suggested-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > You need to be careful with using this.  There are some things below
> > that go beyond what I have suggested.
>
> I think we all suggested some bits and pieces so I included everyone.
> We can drop if there are any objections.

There are, from me and from Andy.

[cut]

> > Up to this point it is all fine IMV.
> >
> > > +/**
> > > + * acpi_dev_uid_match - Match device by supplied UID
> > > + * @adev: ACPI device to match.
> > > + * @uid2: Unique ID of the device.
> > > + *
> > > + * Matches UID in @adev with given @uid2.
> > > + *
> > > + * Returns: %true if matches, %false otherwise.
> > > + */
> > > +
> > > +/* Treat uid as a string for array and pointer types, treat as an integer otherwise */
> > > +#define get_uid_type(x) \
> > > +       (__builtin_choose_expr(is_array_or_pointer_type(x), (const char *)0, (u64)0))
> >
> > But I wouldn't use the above.
> >
> > It is far more elaborate than needed for this use case and may not
> > actually work as expected.  For instance, why would a pointer to a
> > random struct type be a good candidate for a string?
>
> Such case will not compile, since its data type will not match with
> acpi_str_uid_match() prototype. The compiler does a very good job of
> qualifing only the compatible string types here, which is exactly what
> we want.
>
> error: passing argument 2 of 'acpi_str_uid_match' from incompatible pointer type [-Werror=incompatible-pointer-types]
>     if (acpi_dev_uid_match(adev, adev)) {
>                                  ^
> ./include/acpi/acpi_bus.h:870:20: note: expected 'const char *' but argument is of type 'struct acpi_device *'
>  static inline bool acpi_str_uid_match(struct acpi_device *adev, const char *uid2)

You are right, it won't compile, but that's not my point.  Why would
it be matched with acpi_str_uid_match() in the first place?

> > > +
> > > +#define acpi_dev_uid_match(adev, uid2)                         \
> > > +       _Generic(get_uid_type(uid2),                            \
> > > +                const char *: acpi_str_uid_match,              \
> > > +                u64: acpi_int_uid_match)(adev, uid2)
> > > +
> >
> > Personally, I would just do something like the following
> >
> > #define acpi_dev_uid_match(adev, uid2) \
> >         _Generic((uid2), \
> >                 const char *: acpi_str_uid_match, \
> >                 char *: acpi_str_uid_match, \
> >                 const void *: acpi_str_uid_match, \
> >                 void *: acpi_str_uid_match, \
> >                 default: acpi_int_uid_match)(adev, uid2)
> >
> > which doesn't require compiler.h to be fiddled with and is rather
> > straightforward to follow.
> >
> > If I'm to apply the patches, this is about the level of complexity you
> > need to target.
>
> Understood, however this will limit the type support to only a handful
> of types,

Indeed.

> and will not satisfy a few of the existing users, which, for
> example are passing signed or unsigned pointer or an array of u8.

Fair enough, so those types would need to be added to the list.

> Listing every possible type manually for _Generic() looks a bit verbose
> for something that can be simply achieved by __builtin functions in my
> opinion.

But then you don't even need _Generic(), do you?

Wouldn't something like the below work?

#define acpi_dev_uid_match(adev, uid2) \
        (__builtin_choose_expr(is_array_or_pointer_type((uid2)),acpi_str_uid_match(adev,
uid2), acpi_int_uid_match(adev, uid2))

In any case, I'm not particularly convinced about the
is_array_or_pointer_type() thing and so I'm not going to apply the
series as is.