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Message-ID: <202002201541.1B5347ABAD@keescook>
Date: Thu, 20 Feb 2020 15:42:11 -0800
From: Kees Cook <keescook@...omium.org>
To: Jani Nikula <jani.nikula@...ux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Alexander Potapenko <glider@...gle.com>,
intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915: Distribute switch variables for initialization
On Thu, Feb 20, 2020 at 12:21:14PM +0200, Jani Nikula wrote:
> On Wed, 19 Feb 2020, Kees Cook <keescook@...omium.org> wrote:
> > Variables declared in a switch statement before any case statements
> > cannot be automatically initialized with compiler instrumentation (as
> > they are not part of any execution flow). With GCC's proposed automatic
> > stack variable initialization feature, this triggers a warning (and they
> > don't get initialized). Clang's automatic stack variable initialization
> > (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
> > doesn't initialize such variables[1]. Note that these warnings (or silent
> > skipping) happen before the dead-store elimination optimization phase,
> > so even when the automatic initializations are later elided in favor of
> > direct initializations, the warnings remain.
> >
> > To avoid these problems, move such variables into the "case" where
> > they're used or lift them up into the main function body.
> >
> > drivers/gpu/drm/i915/display/intel_display.c: In function ‘check_digital_port_conflicts’:
> > drivers/gpu/drm/i915/display/intel_display.c:12963:17: warning: statement will never be executed [-Wswitch-unreachable]
> > 12963 | unsigned int port_mask;
> > | ^~~~~~~~~
> >
> > drivers/gpu/drm/i915/intel_pm.c: In function ‘vlv_get_fifo_size’:
> > drivers/gpu/drm/i915/intel_pm.c:474:7: warning: statement will never be executed [-Wswitch-unreachable]
> > 474 | u32 dsparb, dsparb2, dsparb3;
> > | ^~~~~~
> > drivers/gpu/drm/i915/intel_pm.c: In function ‘vlv_atomic_update_fifo’:
> > drivers/gpu/drm/i915/intel_pm.c:1997:7: warning: statement will never be executed [-Wswitch-unreachable]
> > 1997 | u32 dsparb, dsparb2, dsparb3;
> > | ^~~~~~
> >
> > [1] https://bugs.llvm.org/show_bug.cgi?id=44916
> >
> > Signed-off-by: Kees Cook <keescook@...omium.org>
>
> Reviewed-by: Jani Nikula <jani.nikula@...el.com>
Thanks!
>
> If you look at i915/Makefile, you'll see that we don't shy away from
> enabling lots of extra warnings, and we run our CI with -Werror to keep
> it clean. It does not seem like -Wswitch-unreachable does me any good,
> though... is it new?
It's already enabled by default, but the GCC feature that tweaks it
doesn't exist yet. But it points out a problem that exists for Clang
today, but Clang doesn't actually warn on (yet). So this is a fix to
avoid the silent Clang problem and fix future warnings before they
happen.
-Kees
>
> BR,
> Jani.
>
>
> > ---
> > drivers/gpu/drm/i915/display/intel_display.c | 6 ++++--
> > drivers/gpu/drm/i915/intel_pm.c | 4 ++--
> > 2 files changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> > index 064dd99bbc49..c829cd26f99e 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > @@ -12960,14 +12960,15 @@ static bool check_digital_port_conflicts(struct intel_atomic_state *state)
> > WARN_ON(!connector_state->crtc);
> >
> > switch (encoder->type) {
> > - unsigned int port_mask;
> > case INTEL_OUTPUT_DDI:
> > if (WARN_ON(!HAS_DDI(to_i915(dev))))
> > break;
> > /* else, fall through */
> > case INTEL_OUTPUT_DP:
> > case INTEL_OUTPUT_HDMI:
> > - case INTEL_OUTPUT_EDP:
> > + case INTEL_OUTPUT_EDP: {
> > + unsigned int port_mask;
> > +
> > port_mask = 1 << encoder->port;
> >
> > /* the same port mustn't appear more than once */
> > @@ -12976,6 +12977,7 @@ static bool check_digital_port_conflicts(struct intel_atomic_state *state)
> >
> > used_ports |= port_mask;
> > break;
> > + }
> > case INTEL_OUTPUT_DP_MST:
> > used_mst_ports |=
> > 1 << encoder->port;
> > diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> > index bd2d30ecc030..17d8833787c4 100644
> > --- a/drivers/gpu/drm/i915/intel_pm.c
> > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > @@ -469,9 +469,9 @@ static void vlv_get_fifo_size(struct intel_crtc_state *crtc_state)
> > struct vlv_fifo_state *fifo_state = &crtc_state->wm.vlv.fifo_state;
> > enum pipe pipe = crtc->pipe;
> > int sprite0_start, sprite1_start;
> > + u32 dsparb, dsparb2, dsparb3;
> >
> > switch (pipe) {
> > - u32 dsparb, dsparb2, dsparb3;
> > case PIPE_A:
> > dsparb = I915_READ(DSPARB);
> > dsparb2 = I915_READ(DSPARB2);
> > @@ -1969,6 +1969,7 @@ static void vlv_atomic_update_fifo(struct intel_atomic_state *state,
> > const struct vlv_fifo_state *fifo_state =
> > &crtc_state->wm.vlv.fifo_state;
> > int sprite0_start, sprite1_start, fifo_size;
> > + u32 dsparb, dsparb2, dsparb3;
> >
> > if (!crtc_state->fifo_changed)
> > return;
> > @@ -1994,7 +1995,6 @@ static void vlv_atomic_update_fifo(struct intel_atomic_state *state,
> > spin_lock(&uncore->lock);
> >
> > switch (crtc->pipe) {
> > - u32 dsparb, dsparb2, dsparb3;
> > case PIPE_A:
> > dsparb = intel_uncore_read_fw(uncore, DSPARB);
> > dsparb2 = intel_uncore_read_fw(uncore, DSPARB2);
> >
>
> --
> Jani Nikula, Intel Open Source Graphics Center
--
Kees Cook
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