linux-kernel - Re: [PATCH] drm/i915,agp/intel: Do not clear stolen entries

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Date:	Mon, 24 Jan 2011 10:10:20 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Arnd Bergmann <arnd@...db.de>, Jiri Olsa <jolsa@...hat.com>,
	Chris Clayton <chris2553@...glemail.com>
Subject: Re: [PATCH] drm/i915,agp/intel: Do not clear stolen entries

On Sun, 23 Jan 2011 23:40:41 -0800 (PST), Hugh Dickins <hughd@...gle.com> wrote:
> It improved matters for me (on a two-year-old Aspire One which had been
> showing the same few characters of text repeated a large number of times
> across the screen with 2.6.38-rc1 and rc2): the VESA framebuffer showing
> good text at last.  But crashed once I tried startx, netconsole showing:

[snip]
 
> But your comment above on clear_range was very helpful: your latest
> patch fixed one call, but left two others unfixed.  Please fold in:
> 
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c	2011-01-23 11:52:47.350395154 -0800
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c	2011-01-23 20:13:01.457805176 -0800
> @@ -36,7 +36,7 @@ void i915_gem_restore_gtt_mappings(struc
>  
>  	/* First fill with scratch pages */
>  	intel_gtt_clear_range(dev_priv->mm.gtt_start / PAGE_SIZE,
> -			      dev_priv->mm.gtt_end / PAGE_SIZE);
> +		(dev_priv->mm.gtt_end - dev_priv->mm.gtt_start) / PAGE_SIZE);
>  
>  	list_for_each_entry(obj, &dev_priv->mm.gtt_list, gtt_list) {
>  		i915_gem_clflush_object(obj);
> --- a/drivers/gpu/drm/i915/i915_gem.c	2011-01-23 11:52:47.346395154 -0800
> +++ b/drivers/gpu/drm/i915/i915_gem.c	2011-01-23 20:10:58.081193280 -0800
> @@ -149,7 +149,7 @@ void i915_gem_do_init(struct drm_device
>  	dev_priv->mm.mappable_gtt_total = min(end, mappable_end) - start;
>  
>  	/* Take over this portion of the GTT */
> -	intel_gtt_clear_range(start / PAGE_SIZE, end / PAGE_SIZE);
> +	intel_gtt_clear_range(start / PAGE_SIZE, (end - start) / PAGE_SIZE);
>  }

The patch I finally applied did include those fixes. After making the
mistake once, I had to double check the other call sites.

> On this laptop I'm typing from (GM965 with KMS), I've had no trouble
> getting X up; but when typing in one of the xterms, typed characters
> often stop echoing, until I shift to a different window, whereupon
> they appear.  This condition cleared (for a while) by switching to
> VESA fb console and back; no such problem observed on that console.
> 
> Does that sound familiar?  I have no evidence whatever that i915 is
> to blame here.  Several times I tried bisecting last week, but each
> attempt ended up in a nonsensical place, because the effect does not
> occur to order.  So I'd sometimes mark a bisection point as good when
> I guess it must actually have been bad.  Perhaps it's a matter of
> timing or an uninitialized variable.  But while I'm here, worth asking
> if that behaviour sounds like anything you might be responsible for?

Sounds suspiciously like the batch buffer is not being dispatched and
flushed to the scanout. A very similar bug was recently fixed for
xf86-video-intel 2.14.0 which was causing deferred output. 
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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