Hi Maarten,
sorry for delay in reply...
In Option 3:
We know maximum number of plane for any given CRTC, We also know, what
is the maximum downscaling supported (only downscaling affects WM) per
pipe/plane.
Maximum downscaling per plane can be :
max plane hscale * max plane vscale, which is 2.99x2.99
in GEN9
This scaling should also be less than cdclk / pixel clock.
same limitation applies for pipe downscaling as well.
following patch implements limitation related to cdclk/pixel_clock
(max supported pixel rate).
https://patchwork.freedesktop.org/patch/141210/
So our downscaling related final limitation will be something like
min ( (max_plane_hscale * max_plane_vscale) *
(max_pipe_hscale * max_pipe_vscale), (cdclk / pixel_clock))
min (2.99*2.99*2.99*2.99, (cdclk / pixel_clock))
During modeset we can compute the same & enable the WA.
One of mem bandwidth limitation is, if Y_tile is enabled in any of the
plane & total display bandwidth is > 20% then enable Y-tile specific WA,
20% mark will hit only in case of DRAM connected is of lower frequency
OR high resolution & high refresh-rate monitors are connected.
for X-tile WA this % is 35% OR 60%, So we have pretty slim chances of
hitting the situation.
for e.g. 4K@60 display will have pixel clock about 540-545MHz, & cdclk
will be 594MHz
if 1600MHz dual-channel DRAM is connected to the system, then available
system bandwidth will be :
1600 * 2 * 8 = 25600,
if 3 planes are enabled & all 3 pipes are enables in that case total
display bandwidth requirement will be approx
545 * 3 * 3 = 4905, which is roughly 20% (19.16%) of total
available bandwidth, & y-tile WA maybe needed
if downscaling is enabled max supported downscaling will be (594 / 545)
1.08%,
in such case max display bandwidth requirement may reach
545 * 1.08 * 3 * 3 = 5297.4, which is 20.69%, & Y-tile WA will be
needed.
for higher frequency DRAM this % will be even less
so whenever total bandwidth is going > 20% & Y-tile is enabled, then
only we may need to take the mutex of all CRTC, so there will be fairly
less changes of holding any lock.
Regards,
-Mahesh
On Tuesday 28 March 2017 01:38 PM, Maarten Lankhorst wrote:
Op 27-03-17 om 17:52 schreef Mahesh Kumar:
*Arbitrated system bandwidth workarounds for watermark.*
All GEN-9 based platforms require watermark related WA to be enabled
if Display memory bandwidth requirement is exceeding XX% of total
available system memory bandwidth.
This XX% depend on multiple factors.
*e.g.* if all the enabled planes have X-tiled or linear memory then,
XX = 60
if any Y-tiled plane is enabled then
XX = 20 etc.
In current implementation of workarounds we enable maximum WA (i.e.
add 15us latency during WM calculation) irrespective of workaround is
required OR not.
total display bandwidth requirement is sum of display requirement of
individual pipe, In order to calculate correct BW requirement plane
configuration of any pipe should not be changing during calculation.
To implement & optimize above requirement many implementations are
possible, I'm proposing few of options.
Please review & let know which option is better to implement WA's.
*Option 1:*
Use connection_mutex (this will change to i915 specific lock only
that is available in atomic design) to serialize all the commits.
If memory bandwidth WA is changing then get all crtc_states for
calculating watermark values.
*Pros:*
* In each flip optimum WM values (not more than the required
value) will be used.
*Cons:*
* This approach will serialize all the flips so there will be
performance impact, in case of blocking commits this impact
will be even worse, e.g. three display with refresh-rate of
30fps, 60fps & 90fps.
* If commit is going-on in 30FPS display, all other flip will
be blocked & frames in 60 & 90fps display will be
dropped/blocked.
*Option 2:*
Use two levels of system bandwidth check, once during calculation
& second during commit.
During intel_atomic_check (as part of compute_ddb) don’t hold any
system level mutex, instead hold WM mutex & compute system
bandwidth requirement. If WA is changing then get crtc_state of
all other pipes & go ahead with commit.
During intel_atomic_commit, again take wm_mutex & recalculate
complete system bandwidth requirement. If requirement is changed
in a way that computed WM are not valid anymore fail the flip.
Update the bandwidth requirement for each plane in global state
(dev_priv->wm) so other flips don’t need to recalculate it.
*Pros:*
* It reduces critical section time.
* Still optimum use of available DDB & optimum WM values are used.
*Cons:*
* If memory bandwidth WA are changing very frequently then
there will be many flip failures which will impact the
performance.
*Option 3:*
Compute maximum bandwidth requirement during modeset.
i.e. if modeset is of 1080p @60fps & maximum plane in CRTC are
3, with maximum supported downscale amount “XX.YY” (defined by
min of cdclk/crtc_clock & max(hscale x vscale)) then max
bandwidth requirement for CRTC will be
(1080p x 60 x 3 x XX.YY).
Now during flip if there is any change which will change the WA
(e.g. tiling change) then take wm_mutex lock & recalculate
complete bandwidth requirement. If WA is changing then get
crtc_state of all other pipes & go ahead with commit. (if total
display memory BW % is less than lowest % to enable WA i.e. 20%,
then no need to recompute)
Update per-CRTC bandwidth requirement in global state so other
flips don’t need to recalculate each time.
*Pros:*
* All CRTC can flip independently until there is change which
will impact WA.
* No locking until potential WM WA change.
*Cons:*
* If memory bandwidth WA is changing very frequently then there
will be slight performance impact.
* We may not be programming optimum WM values, which may have
some power impact.
If you think any other approach should be used please let know that
as well.
Option 4:
Check if watermarks for the current pipe needs global adjustment
between last commit and current, if not do nothing.
If there is, we could do 1 of the below:
1. Blindly grab all other crtc state and do watermark
reprogramming.
2. If it does need adjustment, grab all other crtc's mutexes
and see if we need to adjust watermark state. If we do, grab other affected
crtc's states as well to perform watermark reprogramming.
Perhaps add some elements of option 3 too? I like that one too.
~Maarten
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