Nouveau - Jul 2015 - [Bug 91236] New: [NVC3] Passive DP to HDMI adapter cannot use pixel clocks greater than 165 MHz

https://bugs.freedesktop.org/show_bug.cgi?id=91236

            Bug ID: 91236
           Summary: [NVC3] Passive DP to HDMI adapter cannot use pixel
                    clocks greater than 165 MHz
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/nouveau
          Assignee: nouveau at lists.freedesktop.org
          Reporter: a.boettcher at gmail.com
        QA Contact: xorg-team at lists.x.org

Created attachment 116968
  --> https://bugs.freedesktop.org/attachment.cgi?id=116968&action=edit
dmesg of full boot + setting external display to mode with 297 MHz clock

Starting from stock linux 4.1.0 source, libdrm 2.4.61,
xf86-video-nouveau-1.0.11, xorg-server 1.17.1. The hardware is a Quatro 2000M
in a Lenovo W520 laptop setup as an offload sink and output source for an
integrated intel display. The external display is a Seiki SE39UY04 connected by
HDMI to display port passive adapter.

We attempted to use a 3840x2160 at 30 mode which has a pixel clock of 297 MHz.
This is known to work fine in both the nvidia blob and the windows driver, so
the hardware has no issues driving this clock in a single TDMS link.

First the connector detection code will reject this mode from the default list
because the maximum TDMS speed is based on pre-HDMI 1.3 spec. Second even if
that is worked around the mode is higher than 165 MHz, so the display port
connection will enter dual link mode, which must be stopped in this case.

We attempted to allow the mode to pass through to see if it might just work.
There were two changes:

drivers/gpu/drm/nouveau/nouvau_connector.c modified get_tmds_link_bandwidth()
to return 340000 (HDMI 1.3 spec)

drivers/gpu/drm/nouveau/nvkm/engine/disp/nv50.c commented out line 1547-1548 to
not use dual link mode.

The result is a failure. The screen flickers and does not show anything. A full
dmesg log with nouveau.debug=trace is attached.

I am a C programmer but I have no familiarity with any part of the nouveau or
DRM parts of the world. Very willing to help, just don't know where to
proceed.

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--- Comment #1 from Ilia Mirkin <imirkin at alum.mit.edu> ---
Look for the series starting with

[PATCH 0/2] drm/nouveau: add support for 2560x1440 at 56 over HDMI

sent to nouveau and dri-devel on Aug 8. It limits things to 225mhz though. We
did some poking around with various VBIOS's, I think I found the place where
the max freq is stored, but would be curious to see yours (can be grabbed from
/sys/kernel/debug/dri/0/vbios.rom).

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--- Comment #2 from Andrew Boettcher <a.boettcher at gmail.com> ---
Created attachment 119057
  --> https://bugs.freedesktop.org/attachment.cgi?id=119057&action=edit
Video Bios

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--- Comment #3 from Ilia Mirkin <imirkin at alum.mit.edu> ---
(In reply to Andrew Boettcher from comment #2)
> Created attachment 119057 [details]
> Video Bios
Blast! According to my theory you'd only have 225MHz on this card.
You're sure
about 297MHz working? FTR my theory is that the bit 'T' table contains
this
info, which with your vbios is:

BIT table 'T' version 1 at 0x02d8 length 0x0002
Unknown BIT table 'T' version 1
0x02d8: 41 b5

0000b540  71 20 0d 04 00 00 50 32  74 40 e8 80 e4 57 5b fd  |q ....P2t at
...W[.|

0x4074 = 16500 (single-link dvi)
0x80e8 = 33000 (dual-link dvi)
0x57e4 = 22500 -- my theory for max hdmi link speed

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--- Comment #4 from Andrew Boettcher <a.boettcher at gmail.com> ---
(In reply to Ilia Mirkin from comment #3)
> Blast! According to my theory you'd only have 225MHz on this card.
You're
> sure about 297MHz working?
I have a passive HDMI connector and I can get a 297 mhz mode from the binary
blob as well as in windows over that connector. So they are either doing
something special behind the scenes or that table is not the whole truth.

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--- Comment #5 from Ilia Mirkin <imirkin at alum.mit.edu> ---
With the below pair of patches, you should be able to set any max pixel clock
you like.

http://cgit.freedesktop.org/~darktama/nouveau/commit/?id=8d5fe3328855063bb41acb00b1049138acce8e2e
http://cgit.freedesktop.org/~darktama/nouveau/commit/?id=f787ef9b44102ed01d4255a653d0261bb6365ba3

These should apply relatively easily onto a 4.3 or even older kernel. Boot with
nouveau.hdmimhz=297.

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--- Comment #6 from Andrew Boettcher <a.boettcher at gmail.com> ---
I am reporting the patches worked great! Indeed I am now viewing this bug with
a screen in 297 MHz over a passive DP to HDMI connector. Before we missed the
second part where the dual link mode was activated.

This is great, thanks! I guess the next step is to figure out how to detect the
real limit, which I believe is 300 MHz on this chip.

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Martin Peres <martin.peres at free.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |MOVED

--- Comment #7 from Martin Peres <martin.peres at free.fr> ---
-- GitLab Migration Automatic Message --

This bug has been migrated to freedesktop.org's GitLab instance and has been
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