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Subject: Re: [PATCH 10/13] powerpc/rtas: improve function information lookups
From: Andrew Donnellan <ajd@linux.ibm.com>
To: Nathan Lynch <nathanl@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
Date: Tue, 29 Nov 2022 18:23:09 +1100
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On Mon, 2022-11-28 at 18:08 -0600, Nathan Lynch wrote:
> Andrew Donnellan <ajd@linux.ibm.com> writes:
> > On Fri, 2022-11-18 at 09:07 -0600, Nathan Lynch wrote:
> > > Convert rtas_token() to use a lockless binary search on the
> > > function
> > > table. Fall back to the old behavior for lookups against names
> > > that
> > > are not known to be RTAS functions, but issue a warning.
> > > rtas_token()
> > > is for function names; it is not a general facility for accessing
> > > arbitrary properties of the /rtas node. All known misuses of
> > > rtas_token() have been converted to more appropriate of_ APIs in
> > > preceding changes.
> >=20
> > For in-kernel users, why not go all the way: make rtas_token()
> > static
> > and use it purely for the userspace API,
>=20
> Not sure what userspace API refers to here, can you be more specific?
> I
> don't think rtas_token() is exposed to user space.

Right, what I actually meant to refer to here is the fact that sys_rtas
takes a token, but when I wrote this I forgot that userspace doesn't
pass a string, rather librtas implements rtas_token itself using the
/proc interface to the device tree.

>=20
> > and switch kernel users over
> > to using rtas_function_index directly?
>=20
> Well, I have work in progress to transition all rtas_token() users to
> a
> different API, but using opaque symbolic handles rather than exposing
> the indexes. Something like:
>=20
> /*
> =C2=A0* Opaque handle for client code to refer to RTAS functions. All
> valid
> =C2=A0* function handles are build-time constants prefixed with RTAS_FN_.
> =C2=A0*/
> typedef struct {
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0enum rtas_function_index =
index;
> } rtas_fn_handle_t;
>=20
> #define rtas_fn_handle(x_) (const rtas_fn_handle_t) { .index =3D
> rtas_fnidx(x_), }
>=20
> #define RTAS_FN_CHECK_EXCEPTION=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0
> rtas_fn_handle(CHECK_EXCEPTION)
> #define RTAS_FN_DISPLAY_CHARACTER=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0
> rtas_fn_handle(DISPLAY_CHARACTER)
> #define RTAS_FN_EVENT_SCAN=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0
> rtas_fn_handle(EVENT_SCAN)
>=20
> ...
>=20
> /**
> =C2=A0* rtas_function_token() - RTAS function token lookup.
> =C2=A0* @handle: Function handle, e.g. RTAS_FN_EVENT_SCAN.
> =C2=A0*
> =C2=A0* Context: Any context.
> =C2=A0* Return: the token value for the function if implemented by this
> platform,
> =C2=A0*=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 otherwise RTAS_UN=
KNOWN_SERVICE.
> =C2=A0*/
> s32 rtas_function_token(const rtas_fn_handle_t handle)
> {
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0const size_t index =3D ha=
ndle.index;
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0const bool out_of_bounds =
=3D index >=3D
> ARRAY_SIZE(rtas_function_table);
>=20
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (WARN_ONCE(out_of_boun=
ds, "invalid function index %zu",
> index))
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0return RTAS_UNKNOWN_SERVICE;
>=20
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0return rtas_function_tabl=
e[index].token;
> }
>=20
> But that's a tree-wide change that would touch various drivers, and I
> feel like the current series is what I can reasonably hope to get
> applied right now.

It's not clear to me what the benefit of adding this additional layer
of indirection would be.


--=20
Andrew Donnellan    OzLabs, ADL Canberra
ajd@linux.ibm.com   IBM Australia Limited