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Ville Syrjälä 8ec7d10a54 drm/i915: Fix pre-skl DP AUX precharge length
DP v1.1+ says:
"The DisplayPort transmitter, which is the driving end for a request
 transaction, pre-charges the AUX-CH+ and AUX-CH- to a common mode
 voltage by transmitting 10 to 16 consecutive 0’s in Manchester II code.
 After the active pre-charge, the transmitter sends an AUX Sync pattern.
 The AUX Sync pattern must be as follows:
 Start with 16 consecutive 0s in Manchester-II code, which results in
 a transition from low to high in the middle of each bit period.
 Including active pre-charge pulses, there shall be 26 to 32
 consecutive 0s before the end of the AUX_SYNC pattern."

BDW bspec says:
"Used to determine the precharge time for the Aux Channel. During this
 time the Aux Channel will drive the SYNC pattern. Every microsecond
 gives one additional SYNC pulse beyond the hard coded 26 SYNC pulses.
 The value is the number of microseconds times 2. Default is 3 decimal
 which gives 6us of precharge which is 6 extra SYN pulses for a total
 of 32."

CPT bspec says the same thing apart from:
"... Default is 5 decimal which gives 10us of precharge which is 10
 extra SYNC pulses for a total of 36."

So it looks like to match the max of 32 of the DP spec we should just
always program this extra precharge time to 3.

Unfortunately g4x/ibx bspec doesn't have this clarification, but
since the cpt default was still the same 5 as for g4x/ibx let's
assume the behaviour was always the same.

I also did a bit more archaeology and found the following:
 commit e3421a1894 ("drm/i915: enable DP/eDP for Sandybridge/Cougarpoint")
  added the precharge==3 for snb
 commit 092945e11c ("drm/i915/dp: Use auxch precharge value of 5 everywhere")
  tried to change it to be 5 for snb
 commit 6b4e0a93ff ("Revert "drm/i915/dp: Use auxch precharge value of 5 everywhere"")
  went back to 3 for snb due to a regression

So I think the value of 5 was just always wrong, but I guess very
few display actually get upset if we do too many SYNCs. Also DP 1.0
did not specify any max value for this, whereas DP 1.1+ added the
max==32 wording.

Additionally I hooked up a scope to a few machines with the following
findings:
- ibx and cpt both give us the expected 32 total sync pulses with
  precharge==3
- ctg is a bit different, it has the 10 hardcoded precharge sync
  pulses same as later platforms (so we get at least 26 sync
  pulses in total). However the additional precharge length (which
  is what we're changing here) is not done with sync pulses.
  Instead ctg does this part of the precharge with a steady DC
  voltage. If we wanted to 100% match DP 1.1+ here we should perhaps
  set prechange length to 0, but less precharge might make AUX less
  reliable, and so far we're not aware of any problems due to the DC
  precharge. Hence I think precharge==3 is probably the best choice
  here too to make the total length of precharge consistent with
  the later platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318181039.17260-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>
2021-04-28 15:52:23 +03:00
arch Merge tag 'drm-intel-next-2021-03-16' of git://anongit.freedesktop.org/drm/drm-intel into drm-next 2021-03-18 08:06:34 +10:00
block block: Discard page cache of zone reset target range 2021-03-11 11:49:25 -07:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto crypto: mips/poly1305 - enable for all MIPS processors 2021-03-08 11:52:17 +01:00
Documentation Mediatek DRM Next for Linux 5.13 2021-04-08 12:42:14 +10:00
drivers drm/i915: Fix pre-skl DP AUX precharge length 2021-04-28 15:52:23 +03:00
fs Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
include drm: Rename DP_PSR_SELECTIVE_UPDATE to better mach eDP spec 2021-04-26 13:08:04 -07:00
init Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel Tag for the input subsystem to pick up 2021-03-29 20:35:13 +09:00
lib drm-misc-next for 5.13: 2021-03-16 17:08:46 +10:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
net NFS Client Bugfixes for Linux 5.10-rc3 2021-03-12 14:19:35 -08:00
samples Merge git://git.kernel.org:/pub/scm/linux/kernel/git/netdev/net 2021-03-09 17:15:56 -08:00
scripts drm-misc-next for 5.13: 2021-03-16 17:08:46 +10:00
security Revert 95ebabde38 ("capabilities: Don't allow writing ambiguous v3 file capabilities") 2021-03-12 15:27:14 -06:00
sound ALSA: hda/hdmi: Cancel pending works before suspend 2021-03-10 12:52:01 +01:00
tools A single objtool fix to handle the PUSHF/POPF validation correctly for the 2021-03-14 13:15:55 -07:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Mediatek DRM Next for Linux 5.13 2021-04-08 12:42:14 +10:00
Makefile Linux 5.12-rc3 2021-03-14 14:41:02 -07:00
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