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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
arch ARM: SoC fixes 2020-03-08 17:36:22 -07:00
block block, bfq: fix overwrite of bfq_group pointer in bfq_find_set_group() 2020-03-06 07:00:58 -07:00
certs certs: Add wrapper function to check blacklisted binary hash 2019-11-12 12:25:50 +11:00
crypto Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2020-02-20 15:15:16 -08:00
Documentation ARM: SoC fixes 2020-03-08 17:36:22 -07:00
drivers drm/dp_mst: Use full_pbn instead of available_pbn for bandwidth checks 2020-03-12 19:07:24 -04:00
fs Driver core / debugfs fixes for 5.6-rc5 2020-03-08 10:39:40 -05:00
include drm/dp_mst: Use full_pbn instead of available_pbn for bandwidth checks 2020-03-12 19:07:24 -04:00
init Tracing updates: 2020-02-26 10:34:42 -08:00
ipc Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" 2020-02-21 11:22:15 -08:00
kernel block-5.6-2020-03-07 2020-03-07 14:14:38 -06:00
lib Tracing updates: 2020-02-26 10:34:42 -08:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm mm, hotplug: fix page online with DEBUG_PAGEALLOC compiled but not enabled 2020-03-06 07:06:09 -06:00
net vsock: fix potential deadlock in transport->release() 2020-02-27 12:03:56 -08:00
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scripts parse-maintainers: Mark as executable 2020-03-06 16:29:21 -06:00
security Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2020-02-20 15:15:16 -08:00
sound ASoC: Fixes for v5.6 2020-03-07 07:24:36 +01:00
tools for-linus-2020-03-07 2020-03-07 08:01:43 -06:00
usr Kbuild updates for v5.6 (2nd) 2020-02-09 16:05:50 -08:00
virt KVM/arm fixes for 5.6, take #1 2020-02-28 11:50:06 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
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.gitignore selftest/lkdtm: Use local .gitignore 2020-03-02 08:39:39 -07:00
.mailmap A handful of small documentation fixes that wandered in. 2020-02-07 13:03:10 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Hand MIPS over to Thomas 2020-02-24 22:43:18 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS ARM: SoC fixes 2020-03-08 17:36:22 -07:00
Makefile Linux 5.6-rc5 2020-03-08 17:44:44 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.