On Tue, 2025-01-21 at 17:11 +0200, Ville Syrjälä wrote: > On Tue, Jan 21, 2025 at 10:29:25AM +0000, Hogander, Jouni wrote: > > On Mon, 2025-01-20 at 16:39 +0200, Ville Syrjälä wrote: > > > On Mon, Jan 20, 2025 at 07:28:52AM +0000, Hogander, Jouni wrote: > > > > On Sat, 2025-01-18 at 01:07 +0200, Ville Syrjälä wrote: > > > > > On Fri, Jan 17, 2025 at 10:20:17PM +0200, Ville Syrjälä > > > > > wrote: > > > > > > On Thu, Jan 09, 2025 at 09:31:37AM +0200, Jouni Högander > > > > > > wrote: > > > > > > > Now as we have correct PSR2_MAN_TRK_CTL handling in place > > > > > > > we > > > > > > > can > > > > > > > allow DSB > > > > > > > usage also when PSR is enabled for LunarLake onwards. > > > > > > > > > > > > We seem to still lack an answer as to when the PSR wakes, > > > > > > when > > > > > > it > > > > > > latches the update, and how does all that guarantee that > > > > > > the > > > > > > DSB > > > > > > interrupt fires after the update has been latched? > > > > > > > > > > > > Some thoughts as to how to figure this out: > > > > > > 1. make sure we're in PSR > > > > > > 2. sample TIMESTAMP_CTR > > > > > > 3. start DSB in which > > > > > > write PLANE_SURF with a new value > > > > > > send push > > > > > > wait for vblank > > > > > > poll PLANE_SURFLIVE == new value > > > > > > fire interrupt > > > > > > > > > > > > in the interrupt handler: > > > > > > sample TIMESTAMP_CTR again > > > > > > > > > > > > And then compare flip timestmap vs. frame timestamp vs. the > > > > > > manually sampled timestamps. And then repeat without the > > > > > > SURFLIVE > > > > > > poll to make sure nothing has changed. You'll need to be > > > > > > careful > > > > > > to make sure it will actually poll for long enough to make > > > > > > a > > > > > > real > > > > > > difference (if the poll actuall is needed), but tweaking > > > > > > the > > > > > > poll > > > > > > interval+count suitably. I don't remeber what the max poll > > > > > > count was, but IIRC it wasn't too high so the duration will > > > > > > have > > > > > > to get bumped for long polls. > > > > > > > > > > > > I guess one could also try to poll for the actual PSR > > > > > > status, > > > > > > but dunno how well that'll work. > > > > > > > > > > > > And we could also try to come up with different ideas on > > > > > > where > > > > > > to sample timestamps. Unfortunately we only have the single > > > > > > pipe flip timestamp register so we can only sample one > > > > > > timestamp > > > > > > from the DSB itself per frame. If we had more we could much > > > > > > more > > > > > > easily figure things out :/ > > > > > > > > > > > > I pushed my latest DSB selftest stuff to > > > > > > https://github.com/vsyrjala/linux.git dsb_selftests_7 > > > > > > which has a bunch of stuff for this kind of > > > > > > experimentation. > > > > > > It's in a somewhat sorry state at the moment since I last > > > > > > used > > > > > > to hunt for various DSB bugs, but at least it still builds > > > > > > :) > > > > > > > > > > > > The way I use that is that I run igt 'testdisplay -o ...' > > > > > > to make sure nothing else is actively poking the hardware > > > > > > and then I trigger the DSB tests via debugfs. > > > > > > > > > > I poked around a bit, though only on a TGL+PSR1 system (what > > > > > I > > > > > had > > > > > at hand), so some of this might not apply to PSR2 and/or more > > > > > modern platforms. > > > > > > > > > > General notes: > > > > > - PSR1 exit is triggered by any pipe/plane register write > > > > > (even > > > > > the > > > > > non-arming ones) > > > > > > > > This is same for PSR2 as well. > > > > > > > > > > > > > > We basically have three cases to consider here: > > > > > 1. PSR1 is currently inactive > > > > > Obviously everything should be with the current code, > > > > > vblank evasion works, wait for vblank+interrupt scheme > > > > > for flip completion works > > > > > > > > > > 2. PSR1 is active, but DC states are not > > > > > The wakeup latency here is super quick (it's < 1 scanline, > > > > > how > > > > > much below? I've not yet measured), and arming registers do > > > > > latch > > > > > nearly immediately. > > > > > > Actually it's only ~1usec (based on the timestamps). I also used > > > the > > > following DSB batch to test how many registers we can write > > > there: > > > > > > for (i = 0; ...; i++) > > > dsb_write(PLANE_SURF, i << 12) > > > dsb_interrupt() > > > > > > and then in the interrupt handler I read PLANE_SURFLIVE, and it > > > always > > > shows 0xa000, meaning we only have time to write ten registers. > > > So > > > definitely not enough to guarantee that all arming registers get > > > written. > > > > > > So, for PSR1 at least I think we'd have two options: > > > 1) do a manual PSR wake around the whole commit, which doesn't > > > sound very nice > > > 2) evade hw_scaline==0 so that we wait until we've woken up > > > from the DC state, and then proceed with the normal vblank > > > evasion and arming register writes. And obviously all that > > > only works in DSB_SKIP_WAITS_EN is disabled. Also if all > > > the pipes are doing the full update using the DSB then we > > > could perhaps also remove the explicit DC_OFF dance around > > > the whole commit. > > > > > > > The scanline counter starts counting > > > > > accordingly > > > > > from vblank_start-1. However the hardware still considers > > > > > PSR > > > > > to be > > > > > active for that short duration so DSB_SKIP_WAITS_EN _will_ > > > > > skip > > > > > the > > > > > waits. > > > > > > > > > > Unfortunately being this quick I'm not convinced we have > > > > > enough > > > > > time > > > > > to write all the registers atomically before the hardware > > > > > latches > > > > > something. So I'm thinking we may need to remove > > > > > DSB_SKIP_WAITS_EN, > > > > > in which case the vblank evasion will push the arming > > > > > register > > > > > writes into the next frame. This will mean the wakeup will > > > > > take > > > > > one full frame. > > > > > > > > To my understanding DSB_SKIP_WAITS_EN have impact only when in > > > > SRD > > > > (PSR)/DEEP_SLEEP(PSR2). I.e. In this scenario we still do have > > > > all > > > > waits as in commit without PSR. > > > > > > The PSR state machine is already in link off mode in this case, > > > meaning the pipe has been halted, and the display has been > > > signalled to scan out from its RFB (confirmed by checking the > > > PSR status register in DPCD), and DSB_SKIP_WAITS_EN is already > > > active. But the link is still actually up (I'm guessing it might > > > be transmitting the idle pattern, but I've not confirmed that. > > > Can't remeber if we even have any kind of status register that > > > could show this...). So looks like the link only gets actually > > > turned off by the DMC when entering the DC state. > > > > > > > > > > > > > > > > > 3. PSR1 is active and so are DC states > > > > > The wakeup latency is ~5ms. During that time scanline > > > > > counter > > > > > reads > > > > > 0, PSR is active for the purposes of DSB_SKIP_WAITS_EN. > > > > > Again > > > > > we > > > > > pretty much need DSB_SKIP_WAITS_EN=0 here to make sure the > > > > > interrupt > > > > > gets signalled after the wait for vblank. vblank evasion > > > > > will > > > > > get > > > > > skipped on account the scanline being 0. Somewhat > > > > > ironically > > > > > this > > > > > would give us ~5ms total wakeup latency which is now faster > > > > > than > > > > > the > > > > > previous case. > > > > > > > > Again my understanding is that DSB_SKIP_WAITS_EN=0/1 have > > > > impact > > > > only > > > > when in SRD/DEEP_SLEEP. There is SRD_CTRL/PSR2_CTRL[Idle > > > > Frames] to > > > > control when entering SRD/DEEP_SLEEP. > > > > > > The PSR state machine is what matters. That stops the pipe, and > > > causes > > > DSP_SKIP_WAITS_EN to become active. And the PSR state machine is > > > also > > > what is affected by the idle frames stuff/etc. AFAIK the only > > > stuff > > > that is actually done by the DMC is turning the link/PLL/etc on > > > and > > > off. And obviously link training I guess gets triggered from > > > there > > > somehow when waking up from PSR, hence why the wakeup takes that > > > 5ms > > > longer than when DC States are disabled. > > > > Ok, I need do some investigation on this. The failure I was > > referring > > was this one: > > > > https://intel-gfx-ci.01.org/tree/intel-xe/xe-pw-142521v2/shard-lnl-8/igt@kms_psr2_sf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > > > > I.e. there is that scanline wait at the begin in DSB buffer. It was > > hanging in waiting the scanline. I verified this by checking if > > preceding write to 0x700300 completes and it never did. > > > > This was with PSR2 but not with Panel Replay. Also if I disable > > DEEP_SLEEP by setting PSR2_CTL[idle frames] as 0 it didn't hang. > > I guess that idle frames==0 thing only applies to PSR2. With > PSR1 it doesn't seem to do anything (well, apart from what it > says on the tin). OTOH with PSR1 there is a dedicated link off > vs. standby bit in SRD_CTL, so I guess there is no need for > anything else. > Setting PSR2_CTL[idle frames] as 0 doesn't trigger wake-up. It just disables DEEP_SLEEP. So we can't use that to solve this. I'll guess adding extra CURSURFLIVE write is the way to go with this. BR, Jouni Högander
