Re: Error building v5.5-git on PowerPC32 - bisected to commit 7befe621ff81

Arvind Sankar
2020-02-06T23:12:17Z

On Thu, Feb 06, 2020 at 04:46:52PM -0600, Larry Finger wrote:
> When building post V5.5 on my PowerBook G4 Aluminum, the build failed with the
> following error:
>
> drivers/gpu/drm/drm_edid.c: In function ‘cea_mode_alternate_timings’:
> drivers/gpu/drm/drm_edid.c:3275:2: error: call to ‘__compiletime_assert_3282’
> declared with attribute error: BUILD_BUG_ON failed: cea_mode_for_vic(8)->vtotal
> != 262 || cea_mode_for_vic(9)->vtotal != 262 || cea_mode_for_vic(12)->vtotal !=
> 262 || cea_mode_for_vic(13)->vtotal != 262 || cea_mode_for_vic(23)->vtotal !=
> 312 || cea_mode_for_vic(24)->vtotal != 312 || cea_mode_for_vic(27)->vtotal !=
> 312 || cea_mode_for_vic(28)->vtotal != 312
>
> This error was bisected to commit 7befe621ff81 ("drm/edid: Abstract away
> cea_edid_modes[]").
>
> This problem is clearly a problem with the gcc compiler on the box, namely gcc
> (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3. I had no success finding why the
> attributes were wrong, and finally settled on the following hack:
>
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 99769d6c9f84..062bbe2b254a 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -3272,6 +3272,7 @@ cea_mode_alternate_timings(u8 vic, struct drm_display_mode
> *mode)
> * get the other variants by simply increasing the
> * vertical front porch length.
> */
> +#ifndef CONFIG_PPC32
> BUILD_BUG_ON(cea_mode_for_vic(8)->vtotal != 262 ||
> cea_mode_for_vic(9)->vtotal != 262 ||
> cea_mode_for_vic(12)->vtotal != 262 ||
> @@ -3280,6 +3281,7 @@ cea_mode_alternate_timings(u8 vic, struct drm_display_mode
> *mode)
> cea_mode_for_vic(24)->vtotal != 312 ||
> cea_mode_for_vic(27)->vtotal != 312 ||
> cea_mode_for_vic(28)->vtotal != 312);
> +#endif
>
> if (((vic == 8 || vic == 9 ||
> vic == 12 || vic == 13) && mode->vtotal < 263) ||
>
> Disabling the build check allows me to build and test the post v5.5 versions.
>
> Larry

It's not that the attributes are wrong. The problem is that BUILD_BUG_ON
requires a compile-time evaluatable condition. gcc-4.6 is apparently not
good enough at optimizing to reduce that expression to a constant,
though it was able to do it with the array accesses.