[PATCH] char/agp: Disable frontend without CONFIG_DRM_LEGACY

[Daniel Vetter]

On Wed, Nov 18, 2020 at 9:24 AM Christian König
<christian.koenig at amd.com> wrote:
>
> Am 18.11.20 um 09:02 schrieb Thomas Zimmermann:
> > Hi
> >
> > Am 17.11.20 um 22:40 schrieb Daniel Vetter:
> >> It's probably full of bugs ready for exploiting by userspace. And
> >> there's not going to be any userspace for this without any of the drm
> >> legacy drivers enabled too. So just couple it together.
> >>
> >> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> >> Cc: David Airlie <airlied at linux.ie>
> >> Cc: Adam Jackson <ajax at redhat.com>
> >> ---
> >>   drivers/char/agp/Makefile | 6 +++++-
> >>   drivers/char/agp/agp.h    | 5 +++++
> >>   2 files changed, 10 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/char/agp/Makefile b/drivers/char/agp/Makefile
> >> index cb2497d157f6..90ed8c789e48 100644
> >> --- a/drivers/char/agp/Makefile
> >> +++ b/drivers/char/agp/Makefile
> >> @@ -1,7 +1,11 @@
> >>   # SPDX-License-Identifier: GPL-2.0
> >> -agpgart-y := backend.o frontend.o generic.o isoch.o
> >> +agpgart-y := backend.o generic.o isoch.o
> >>
> >> +ifeq ($(CONFIG_DRM_LEGACY),y)
> >>   agpgart-$(CONFIG_COMPAT)   += compat_ioctl.o
> >> +agpgart-y                   += frontend.o
> >> +endif
> >> +
> >>
> >>   obj-$(CONFIG_AGP)          += agpgart.o
> >>   obj-$(CONFIG_AGP_ALI)              += ali-agp.o
> >> diff --git a/drivers/char/agp/agp.h b/drivers/char/agp/agp.h
> >> index 4eb1c772ded7..bb09d64cd51e 100644
> >> --- a/drivers/char/agp/agp.h
> >> +++ b/drivers/char/agp/agp.h
> >> @@ -186,8 +186,13 @@ int agp_add_bridge(struct agp_bridge_data *bridge);
> >>   void agp_remove_bridge(struct agp_bridge_data *bridge);
> >>
> >>   /* Frontend routines. */
> >> +#if IS_ENABLED(CONFIG_DRM_LEGACY)
> >>   int agp_frontend_initialize(void);
> >>   void agp_frontend_cleanup(void);
> >> +#else
> >> +static inline int agp_frontend_initialize(void) { return 0; }
> >> +static inline void agp_frontend_cleanup(void) {}
> >> +#endif
> > There's one non-legacy driver that uses these agp structures, which is
> > radeon. Does this change affect radeon?
>
> Nouveau uses AGP as well, but I'm not sure if both drivers use any of
> this stuff.

frontend = /dev/agp chardev interface for userspace drivers. If you're
looking at kernel drivers, you're looking at the wrong thing, the
kernel-internal interface is in char/agp/backend.c and still enabled.
So no impact at all on any kernel code.

Now the impact this does have on kms drivers using agp is that there's
no longer a userspace ioctl interface to change the agp setup and
mappings and fight the kms driver (which assumes it's fully in control
of agp configuration).
-Daniel

>
> Regards,
> Christian.
>
> >
> > Best regards
> > Thomas
> >
> >>
> >>   /* Generic routines. */
> >>   void agp_generic_enable(struct agp_bridge_data *bridge, u32 mode);
> >>
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch