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

Emil Velikov
2020-02-20T13:21:17Z

On Wed, 19 Feb 2020 at 20:35, Ville Syrjala
<ville.syrjala@linux.intel.com> wrote:
>
> From: Ville Syrjälä <ville.syrjala@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