linux-kernel - Re: [Intel-gfx] [BUG, bisect] drm/i915: mouse pointer lags and overshoots

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Date:	Mon, 19 Jan 2015 08:40:24 -0800
From:	Matt Roper <matthew.d.roper@...el.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...el.com>,
	Ville Syrjälä 
	<ville.syrjala@...ux.intel.com>,
	Jeremiah Mahler <jmmahler@...il.com>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	David Airlie <airlied@...ux.ie>,
	Alex Deucher <alexander.deucher@....com>,
	Dave Airlie <airlied@...hat.com>,
	Ander Conselvan de Oliveira <conselvan2@...il.com>,
	linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [BUG, bisect] drm/i915: mouse pointer lags and
 overshoots

On Mon, Jan 19, 2015 at 11:04:04AM +0000, Chris Wilson wrote:
> On Mon, Jan 19, 2015 at 11:51:43AM +0100, Daniel Vetter wrote:
> > There's also an issue in (most) X drivers which exaberates this
> > issues: When changing the cursor buffer the X cursor code does a a)
> > disable cursor b) update cursor image c) enable cursor cycle.
> 
> Notably not -intel on which the bug has been observed. And more
> importantly, the slow downs don't seem to correlate with cursor change,
> just cursor movement.
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

It seems that the simple fix for this case (movement only) is to just
skip the prepare_fb/cleanup_fb calls (and the associated vblank wait) in
the transitional plane helper when newfb == oldfb.  I just posted a
small patch that makes that change (and solves the cursor lag for me).

This won't solve the case if userspace uses a different framebuffer for
each update (while trying to update faster than the refresh rate).  Is
there any existing userspace that behaves this way that we can test
with?


Matt

-- 
Matt Roper
Graphics Software Engineer
IoTG Platform Enabling & Development
Intel Corporation
(916) 356-2795
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