The DDB allocation algorithm currently used by the driver grants each
plane a very small minimum allocation of DDB blocks and then divies up
all of the remaining blocks based on the percentage of the total data
rate that the plane makes up. It turns out that this proportional
allocation approach is overly-generous with the larger planes and can
leave very small planes wthout a big enough allocation to even hit their
level 0 watermark requirements (especially on APL, which has a smaller
DDB in general than other gen9 platforms). Or there can be situations
where the smallest planes hit a lower watermark level than they should
have been able to hit with a more equitable division of DDB blocks, thus
limiting the overall system sleep state that can be achieved.
The bspec now describes an alternate algorithm that can be used to
overcome these types of issues. With the new algorithm, we calculate
all plane watermark values for all wm levels first, then go back and
partition a pipe's DDB space second. The DDB allocation will calculate
what the highest watermark level that can be achieved on *all* active
planes, and then grant the blocks necessary to hit that level to each
plane. Any remaining blocks are then divided up proportionally
according to data rate, similar to the old algorithm.
There was a previous attempt to implement this algorithm a couple years
ago in bb9d85f6e9 ("drm/i915/skl: New ddb allocation algorithm"), but
some regressions were reported, the patch was reverted, and nobody
ever got around to figuring out exactly where the bug was in that
version. Our watermark code has evolved significantly in the meantime,
but we're still getting bug reports caused by the unfair proportional
algorithm, so let's give this another shot.
v2:
- Make sure cursor allocation stays constant and fixed at the end of
the pipe allocation.
- Fix some watermark level iterators that weren't handling the max
level.
v3:
- Ensure we don't leave any DDB blocks unused by using DIV_ROUND_UP+min
to calculate the extra blocks for each plane. (Ville)
- Replace a while() loop with a for() loop to be more consistent with
surrounding code. (Ville)
- Clean unattainable watermark levels with memset rather than directly
clearing the member fields. Also do the same for the transition
watermark values if they can't be achieved. (Ville)
- Drop min_disp_buf_needed calculations in skl_compute_plane_wm() since
the results are no longer needed or used. (Ville)
- Drop skl_latency[0] != 0 sanity check; both watermark methods already
account for an invalid 0 latency by returning FP_16_16_MAX. (Ville)
v4:
- Break DDB allocation loop when total_data_rate=0 rather than
alloc_size=0. If total_data_rate has dropped to 0, all remaining
planes are disabled, which isn't true for alloc_size (we might just
have not had any remaining blocks to hand out). Plus
total_data_rate=0 is the case we need to avoid to a prevent a
div-by-0. (Ville)
- s/DIV_ROUND_UP/DIV64_U64_ROUND_UP/ to prevent 32-bit breakage (Ville)
v5:
- Don't forget to move 'start' pointer forward for UV surface when
setting plane DDB boundaries. (Ville)
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105458
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181211173107.11068-2-matthew.d.roper@intel.com
d8e8749802