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Subject: Re: [PATCH 03/13] powerpc/rtas: avoid device tree lookups in
 rtas_os_term()
From: Andrew Donnellan <ajd@linux.ibm.com>
To: Nathan Lynch <nathanl@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
Date: Tue, 22 Nov 2022 14:03:59 +1100
In-Reply-To: <20221118150751.469393-4-nathanl@linux.ibm.com>
References: <20221118150751.469393-1-nathanl@linux.ibm.com>
	 <20221118150751.469393-4-nathanl@linux.ibm.com>
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Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev" <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Fri, 2022-11-18 at 09:07 -0600, Nathan Lynch wrote:
> rtas_os_term() is called during panic. Its behavior depends on a
> couple of conditions in the /rtas node of the device tree, the
> traversal of which entails locking and local IRQ state changes. If
> the
> kernel panics while devtree_lock is held, rtas_os_term() as currently
> written could hang.
>=20
> Instead of discovering the relevant characteristics at panic time,
> cache them in file-static variables at boot. Note the lookup for
> "ibm,extended-os-term" is converted to of_property_read_bool() since
> it is a boolean property, not a RTAS function token.
>=20
> Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>

This seems sensible, minor comment below.

Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>

> ---
> =C2=A0arch/powerpc/kernel/rtas.c | 14 +++++++++++---
> =C2=A01 file changed, 11 insertions(+), 3 deletions(-)
>=20
> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
> index c12dd5ed5e00..81e4996012b7 100644
> --- a/arch/powerpc/kernel/rtas.c
> +++ b/arch/powerpc/kernel/rtas.c
> @@ -947,6 +947,8 @@ void __noreturn rtas_halt(void)
> =C2=A0
> =C2=A0/* Must be in the RMO region, so we place it here */
> =C2=A0static char rtas_os_term_buf[2048];
> +static s32 ibm_os_term_token =3D RTAS_UNKNOWN_SERVICE;

s32 and int are obviously identical, but rtas_token is declared using
int, as are the other variables where we cache various tokens.

> +static bool ibm_extended_os_term;
> =C2=A0
> =C2=A0void rtas_os_term(char *str)
> =C2=A0{
> @@ -958,14 +960,13 @@ void rtas_os_term(char *str)
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * this property may term=
inate the partition which we want to
> avoid
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * since it interferes wi=
th panic_timeout.
> =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=A0if (RTAS_UNKNOWN_SERVICE =3D=
=3D rtas_token("ibm,os-term") ||
> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 RTAS_UNKNOW=
N_SERVICE =3D=3D rtas_token("ibm,extended-os-
> term"))
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (ibm_os_term_token =3D=3D R=
TAS_UNKNOWN_SERVICE ||
> !ibm_extended_os_term)
> =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;
> =C2=A0
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0snprintf(rtas_os_term_buf=
, 2048, "OS panic: %s", str);
> =C2=A0
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0do {
> -=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=A0status =3D rtas_call(rtas_token("ibm,os-term"), 1, 1,
> NULL,
> +=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=A0status =3D rtas_call(ibm_os_term_token, 1, 1, NULL,
> =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=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 __pa(rtas_os_term=
_buf));
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} while (rtas_busy_delay(=
status));
> =C2=A0
> @@ -1335,6 +1336,13 @@ void __init rtas_initialize(void)
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0no_entry =3D of_property_=
read_u32(rtas.dev, "linux,rtas-entry",
> &entry);
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0rtas.entry =3D no_entry ?=
 rtas.base : entry;
> =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 * Discover these now to avoid=
 device tree lookups in the
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * panic path.
> +=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=A0ibm_os_term_token =3D rtas_tok=
en("ibm,os-term");
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ibm_extended_os_term =3D of_pr=
operty_read_bool(rtas.dev,
> "ibm,extended-os-term");
> +
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/* If RTAS was found, all=
ocate the RMO buffer for it and look
> for
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * the stop-self token if=
 any
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */

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