linux/include
Lyude Paul fcf4638075 drm/dp_mst: Use full_pbn instead of available_pbn for bandwidth checks
DisplayPort specifications are fun. For a while, it's been really
unclear to us what available_pbn actually does. There's a somewhat vague
explanation in the DisplayPort spec (starting from 1.2) that partially
explains it:

  The minimum payload bandwidth number supported by the path. Each node
  updates this number with its available payload bandwidth number if its
  payload bandwidth number is less than that in the Message Transaction
  reply.

So, it sounds like available_pbn represents the smallest link rate in
use between the source and the branch device. Cool, so full_pbn is just
the highest possible PBN that the branch device supports right?

Well, we assumed that for quite a while until Sean Paul noticed that on
some MST hubs, available_pbn will actually get set to 0 whenever there's
any active payloads on the respective branch device. This caused quite a
bit of confusion since clearing the payload ID table would end up fixing
the available_pbn value.

So, we just went with that until commit cd82d82cbc ("drm/dp_mst: Add
branch bandwidth validation to MST atomic check") started breaking
people's setups due to us getting erroneous available_pbn values. So, we
did some more digging and got confused until we finally looked at the
definition for full_pbn:

  The bandwidth of the link at the trained link rate and lane count
  between the DP Source device and the DP Sink device with no time slots
  allocated to VC Payloads, represented as a Payload Bandwidth Number. As
  with the Available_Payload_Bandwidth_Number, this number is determined
  by the link with the lowest lane count and link rate.

That's what we get for not reading specs closely enough, hehe. So, since
full_pbn is definitely what we want for doing bandwidth restriction
checks - let's start using that instead and ignore available_pbn
entirely.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: cd82d82cbc ("drm/dp_mst: Add branch bandwidth validation to MST atomic check")
Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: Mikita Lipski <mikita.lipski@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306234623.547525-3-lyude@redhat.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
2020-03-12 19:07:24 -04:00
..
acpi ACPI fixes for 5.6-rc4 2020-02-28 09:02:18 -08:00
asm-generic Microblaze patches for 5.6-rc1 2020-02-04 11:58:07 +00:00
clocksource
crypto
drm drm/dp_mst: Use full_pbn instead of available_pbn for bandwidth checks 2020-03-12 19:07:24 -04:00
dt-bindings ARM: SoC: late updates 2020-02-08 14:17:27 -08:00
keys
kunit
kvm
linux Driver core / debugfs fixes for 5.6-rc5 2020-03-08 10:39:40 -05:00
math-emu
media
misc
net net: sched: correct flower port blocking 2020-02-17 21:33:28 -08:00
pcmcia
ras
rdma RDMA/core: Make the entire API tree static 2020-01-30 16:28:52 -04:00
scsi scsi: Revert "target: iscsi: Wait for all commands to finish before freeing a session" 2020-02-14 17:13:54 -05:00
soc ARM: SoC-related driver updates 2020-02-08 14:04:19 -08:00
sound ASoC: Fixes for v5.6 2020-03-07 07:24:36 +01:00
target
trace ARM: SoC-related driver updates 2020-02-08 14:04:19 -08:00
uapi - Fix request-based DM's congestion_fn and actually wire it up to the 2020-03-04 13:02:45 -06:00
vdso
video
xen xen/xenbus: fix locking 2020-03-05 09:42:23 -06:00