[PATCH 00/12] drm: Put drm_display_mode on diet

[Emil Velikov]

On Wed, 19 Feb 2020 at 20:35, Ville Syrjala
<ville.syrjala at linux.intel.com> wrote:
>
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>
> struct drm_display_mode is extremely fat. Put it on diet.
>
> Some stats for the whole series:
>
> 64bit sizeof(struct drm_display_mode):
> 200 -> 136 bytes (-32%)
>
> 64bit bloat-o-meter -c drm.ko:
> add/remove: 1/0 grow/shrink: 29/47 up/down: 893/-1544 (-651)
> Function                                     old     new   delta
> ...
> Total: Before=189430, After=188779, chg -0.34%
> add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
> Data                                         old     new   delta
> Total: Before=11667, After=11667, chg +0.00%
> add/remove: 0/0 grow/shrink: 0/5 up/down: 0/-16896 (-16896)
> RO Data                                      old     new   delta
> edid_4k_modes                               1000     680    -320
> edid_est_modes                              3400    2312   -1088
> edid_cea_modes_193                          5400    3672   -1728
> drm_dmt_modes                              17600   11968   -5632
> edid_cea_modes_1                           25400   17272   -8128
> Total: Before=71239, After=54343, chg -23.72%
>
>
> 64bit bloat-o-meter drm.ko:
> add/remove: 1/0 grow/shrink: 29/52 up/down: 893/-18440 (-17547)
> ...
> Total: Before=272336, After=254789, chg -6.44%
>
>
> 32bit sizeof(struct drm_display_mode):
> 184 -> 120 bytes (-34%)
>
> 32bit bloat-o-meter -c drm.ko
> add/remove: 1/0 grow/shrink: 19/21 up/down: 743/-1368 (-625)
> Function                                     old     new   delta
> ...
> Total: Before=172359, After=171734, chg -0.36%
> add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
> Data                                         old     new   delta
> Total: Before=4227, After=4227, chg +0.00%
> add/remove: 0/0 grow/shrink: 0/5 up/down: 0/-16896 (-16896)
> RO Data                                      old     new   delta
> edid_4k_modes                                920     600    -320
> edid_est_modes                              3128    2040   -1088
> edid_cea_modes_193                          4968    3240   -1728
> drm_dmt_modes                              16192   10560   -5632
> edid_cea_modes_1                           23368   15240   -8128
> Total: Before=59230, After=42334, chg -28.53%
>
> 32bit bloat-o-meter drm.ko:
> add/remove: 1/0 grow/shrink: 19/26 up/down: 743/-18264 (-17521)
> ...
> Total: Before=235816, After=218295, chg -7.43%
>
>
> Some ideas for further reduction:
> - Convert mode->name to a pointer (saves 24/28 bytes in the
>   struct but would often require a heap alloc for the name (though
>   typical mode name is <10 bytes so still overall win perhaps)
> - Get rid of mode->name entirely? I guess setcrtc & co. is the only
>   place where we have to preserve the user provided name, elsewhere
>   could pehaps just generate on demand? Not sure how tricky this
>   would get.

The series does some great work, with future work reaching the cache
line for 64bit.
Doing much more than that might be an overkill IMHO.

In particular, if we change DRM_DISPLAY_MODE_LEN to 24 we get there,
avoiding the heap alloc/calc on demand fun.
While also ensuring the name is sufficiently large for the next decade or so.

>From drm_mode_set_name():

snprintf(mode->name, DRM_DISPLAY_MODE_LEN, "%dx%d%s",
    mode->hdisplay, mode->vdisplay, interlaced ? "i" : "");


We change the h/v display from (10^15)-1 to (10^11)-1 which seems reasonable.

Note: haven't checked if name includes the terminating \0, so take
numbers with a grain of salt.

-Emil