linux-kernel - Re: [PATCH] drm/i915: Distribute switch variables for initialization

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Date:   Thu, 20 Feb 2020 12:21:14 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915: Distribute switch variables for initialization

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>

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?

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