Bug 91789 - MST (multi stream) with Asus PQ321 drm_dp_calc_pbn_mode() fails
Summary: MST (multi stream) with Asus PQ321 drm_dp_calc_pbn_mode() fails
Alias: None
Product: DRI
Classification: Unclassified
Component: General (show other bugs)
Version: unspecified
Hardware: x86 (IA32) Linux (All)
: medium normal
Assignee: Default DRI bug account
QA Contact:
Depends on:
Reported: 2015-08-28 08:12 UTC by Peter.Blum
Modified: 2016-04-25 12:24 UTC (History)
1 user (show)

See Also:
i915 platform:
i915 features:


Description Peter.Blum 2015-08-28 08:12:55 UTC
If a Asus monitor PQ321 is used in MST mode it gets no signal.
This is because the function drm_dp_calc_pbn_mode() in drm_dp_mst_topology.c returns a bad pixel bandwith number for the mode

Modeline 131:"1920x2160" 60 297000 1920 2008 2052 2200 2160 2168 2178 2250 0x48 0x5

The function return 8 and not the correct value of 1063.

Using simple u32 arithmetic in drm_dp_calc_pbn_mode() the monitor works fine.

I'm not sure if unsigned 32 bit would overflow in some cases.
Comment 1 Jani Nikula 2016-04-25 12:24:46 UTC
Presumed fixed by

commit a9ebb3e46c7ef6112c0da466ef0954673ad36832
Author: Harry Wentland <harry.wentland@amd.com>
Date:   Fri Jan 22 17:07:26 2016 -0500

    drm/dp/mst: Calculate MST PBN with 31.32 fixed point

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