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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Arnd Bergmann <arnd@kernel.org>, linux-kernel@vger.kernel.org,
        Martin Sebor <msebor@gcc.gnu.org>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
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Subject: Re: [PATCH 10/11] drm/i915: avoid stringop-overread warning on pri_latency
In-Reply-To: <20210322160253.4032422-11-arnd@kernel.org>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20210322160253.4032422-1-arnd@kernel.org> <20210322160253.4032422-11-arnd@kernel.org>
Date:   Wed, 24 Mar 2021 17:30:24 +0200
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Precedence: bulk

On Mon, 22 Mar 2021, Arnd Bergmann <arnd@kernel.org> wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> gcc-11 warns about what appears to be an out-of-range array access:
>
> In function ‘snb_wm_latency_quirk’,
>     inlined from ‘ilk_setup_wm_latency’ at drivers/gpu/drm/i915/intel_pm.c:3108:3:
> drivers/gpu/drm/i915/intel_pm.c:3057:9: error: ‘intel_print_wm_latency’ reading 16 bytes from a region of size 10 [-Werror=stringop-overread]
>  3057 |         intel_print_wm_latency(dev_priv, "Primary", dev_priv->wm.pri_latency);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/gpu/drm/i915/intel_pm.c: In function ‘ilk_setup_wm_latency’:
> drivers/gpu/drm/i915/intel_pm.c:3057:9: note: referencing argument 3 of type ‘const u16 *’ {aka ‘const short unsigned int *’}
> drivers/gpu/drm/i915/intel_pm.c:2994:13: note: in a call to function ‘intel_print_wm_latency’
>  2994 | static void intel_print_wm_latency(struct drm_i915_private *dev_priv,
>       |             ^~~~~~~~~~~~~~~~~~~~~~
>
> My guess is that this code is actually safe because the size of the
> array depends on the hardware generation, and the function checks for
> that, but at the same time I would not expect the compiler to work it
> out correctly, and the code seems a little fragile with regards to
> future changes. Simply increasing the size of the array should help.

Agreed, I don't think there's an issue, but the code could use a bunch
of improvements.

Like, we have intel_print_wm_latency() for debug logging and
wm_latency_show() for debugfs, and there's a bunch of duplication and
ugh.

But this seems like the easiest fix for the warning.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>


> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/gpu/drm/i915/i915_drv.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 26d69d06aa6d..3567602e0a35 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1095,11 +1095,11 @@ struct drm_i915_private {
>  		 * in 0.5us units for WM1+.
>  		 */
>  		/* primary */
> -		u16 pri_latency[5];
> +		u16 pri_latency[8];
>  		/* sprite */
> -		u16 spr_latency[5];
> +		u16 spr_latency[8];
>  		/* cursor */
> -		u16 cur_latency[5];
> +		u16 cur_latency[8];
>  		/*
>  		 * Raw watermark memory latency values
>  		 * for SKL for all 8 levels

-- 
Jani Nikula, Intel Open Source Graphics Center