[Bug 106250] Regression with Dell TB16 dock and Linux kernel 4.16.x

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106250

--- Comment #10 from Stanislav Lisovskiy <stanislav.lisovskiy at intel.com> ---
After discussion it was decided that I need to write a summary in order to
escalate this issue further. Problem is that for some architectures Y Tiled
mode goes beyond DDB resources available, especially when used with multiple 4K
displays. 
Currently userspace has no way or procedure to determine and fallback to proper
or at least working display mode, so in some cases, drm_mode_setcrtc fails like
with this bug and we get black screen. 
It was proposed that we might need to fix this in mesa in order to make it be
able to determine if the display mode fits into WM requirements, before it
attempts to do a modeset. One, heuristic way could be to probe, if the
ddb_allocation can fit at least twice as required buffer(most common scenario)
in order to be able to determine if Y Tiled mode is usable, otherwise switch to
X Tiled mode. So basically fixing this might require changes both in user and
kernel space. In kernel space, an ioctl to query the minimum required
ddb_allocation then might be needed.
There is also a proposal to develop, a correspondent stress test case in IGT,
in order to be able to detect similar kind of situations.

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