[10/11] drm/i915: avoid stringop-overread warning on pri_latency

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/gpu/drm/i915/i915_drv.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

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

On Wed, Mar 24, 2021 at 05:30:24PM +0200, Jani Nikula wrote:
> 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.

There is all this ancient stuff in review limbo...
https://patchwork.freedesktop.org/series/50802/