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    SubjectRe: [PATCH] drm/i915,agp/intel: Do not clear stolen entries
    On Fri, Jan 28, 2011 at 6:59 PM, Mario Kleiner
    <> wrote:
    > On Jan 28, 2011, at 11:00 PM, Hugh Dickins wrote:
    >> Sorry, this is now abount vblank or scanout rather than stolen entries.
    >> On Mon, 24 Jan 2011, Chris Wilson wrote:
    >>> On Sun, 23 Jan 2011 23:40:41 -0800 (PST), Hugh Dickins <>
    >>> wrote:
    >>>> 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.
    >> I made a more patient bisection during the week, on x86_64 which
    >> seemed more consistent than i386, and this time it converged sensibly:
    >> to commit 0af7e4dff50454905092d468e91c1ef92e10e6b4
    >> drm/i915: Add support for precise vblank timestamping (v2)
    >> Which kindly notes in its commit message:
    >>    This code has been only tested on a HP-Mini Netbook with
    >>    Atom processor and Intel 945GME gpu. The codepath for
    >>    (IS_G4X(dev) || IS_GEN5(dev) || IS_GEN6(dev)) gpu's
    >>    has not been tested so far due to lack of hardware.
    >> so not surprising that it doesn't work on GM965.
    >> I'm now running with this silly revert:
    >> --- a/drivers/gpu/drm/i915/i915_drv.c   2011-01-18 22:04:29.000000000
    >> -0800
    >> +++ b/drivers/gpu/drm/i915/i915_drv.c   2011-01-24 19:35:51.000000000
    >> -0800
    >> @@ -674,8 +674,8 @@ static struct drm_driver driver = {
    >>        .device_is_agp = i915_driver_device_is_agp,
    >>        .enable_vblank = i915_enable_vblank,
    >>        .disable_vblank = i915_disable_vblank,
    >> -       .get_vblank_timestamp = i915_get_vblank_timestamp,
    >> -       .get_scanout_position = i915_get_crtc_scanoutpos,
    >> +       .get_vblank_timestamp = NULL /* i915_get_vblank_timestamp */,
    >> +       .get_scanout_position = NULL /* i915_get_crtc_scanoutpos */,
    >>        .irq_preinstall = i915_driver_irq_preinstall,
    >>        .irq_postinstall = i915_driver_irq_postinstall,
    >>        .irq_uninstall = i915_driver_irq_uninstall,
    >> which makes 2.6.38-rc usable; though I do believe that I've seen
    >> the same issue (unflushed text) occur a couple of times since, much
    >> too rare to bisect or get upset by, but indicative of some remaining bug.
    > Hi,
    > just skimmed through the archives of this thread. Do i understand correctly
    > that the problem that gets fixed by your revert is that
    > <snip>
    >>>> 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.
    > </snip>

    Yes, that's the problem that's fixed by the little revert patch I
    posted last time.
    Sorry, this thread started out with other problems, then I asked Chris
    if this might also be an i915 issue.

    > Is this with desktop composition enabled?

    Not that I'm aware of. The see-through business. I'm just using four
    xterms in fvwm2 on openSUSE11.2 with own kernel. If desktop
    composition might be enabled by the X startup script, expecting me to
    use gnome rather than fvwm2, then I suppose it might be enabled; but
    it's not something I've chosen to turn on. What should I check to
    answer you for sure, if it matters?

    > Do things like glxgears in a
    > window work correctly? If desktop composition is off?

    Yes, glxgears appears to work correctly: I type "glxgears" at the
    xterm shell prompt, those letters and carriage return are not echoed
    back to me, but the glxgears window appears with the gears turning
    correctly, then I close that window, type more and again my typing is
    not echoed.

    > For a softer fix to the problem you can revert your revert and disable use
    > of those functions by the drm core via:
    > echo 0 > /sys/modules/drm/parameters/timestamp_precision_usec

    Thanks for the info. ("module" rather than "modules".)

    > But can you run it with echo 7 >  /sys/modules/drm/parameters/debug
    > and show me bits of the syslog output when the problem happens? Especially
    > output from the functions "drm_calc_vbltimestamp_from_scanoutpos" and
    > "drm_handle_vblank" and maybe for "vblank_disable_fn",
    > "drm_update_vblank_count", and "drm_vblank_get".

    Wow, millions of lines of output (partly because I couldn't see the
    typo that had prevented me from turning it off after a few seconds).
    I rebuilt the kernel with the DRM_DEBUG at the head of drm_ioctl()
    edited out: that generates so many messages (cmd=0x400c645f
    mostly, but some cmd=0x6458) that the logging cannot keep up, and
    hardly gets a chance to print anything else.

    But even with that edited out, nothing from any of the functions that
    you suggest: only, and perhaps this is the problem?,
    [drm:i915_driver_irq_handler}, pipe a underrun
    about 64 times per second.

    I just tried setting the debug to 7 for a few seconds on 2.6.37, where
    I see no problem: I appear to get the "pipe a underrun" messages with
    that too; and the drm_ioctl messages, but much much fewer of them.
    Though I've been veering between i386 and x86_64 in these tests, so
    keep that in mind if what I'm saying makes no sense: the huge number
    of drm_ioctls was with 2.6.36-rc2 (plus some of Chris's fixes) on
    i386; the 64 underruns per second was with 2.6.36-rc2 (plus some of
    Chris's fixes, minus the drm_ioctl DRM_DEBUG) on x86_64; the underruns
    and reasonable number of drm_ioctls was with 2.6.37 on x86_64.

    On this laptop I'm working with
    # CONFIG_NO_HZ is not set
    # CONFIG_HIGH_RES_TIMERS is not set
    if those have any relevance.


    > Those functions (are supposed to) compute exact timestamps of start of
    > scanout after each vblank. If they get disabled via the "echo 0 ..." then a
    > do_gettimeofday() is called for a crude approximation of start of scanout.
    > The computed timestamps are returned to clients which want them
    > (oml_sync_control extension). I doubt that many apps use that extension or
    > its timestamps already, especially not desktop compositors etc., so i
    > wouldn't expect trouble from such wrong timestamps.
    > However, the timestamps are also used in drm_handle_vblank() in
    > drivers/gpu/drm/drm_irq.c at each vblank irq to detect and filter out
    > redundant vblank irq's to avoid miscounting of vblanks (observed on some
    > Radeon's). If the kms driver would deliver a grossly wrong timestamp and
    > something would be wrong in the implementation of that filtering, it could
    > happen that the vblank counter doesn't get incremented -> delivery of a
    > vblank event to the x-server gets delayed -> a swapbuffer operation on a
    > composited desktop gets delayed -> content of a redirected window updates
    > only with a delay.
    > The relevant check which could prevent vblank counter increments and delay
    > vblank event delivery to the x-server in drm_handle_vblank() would be:
    >        if (abs(diff_ns) > DRM_REDUNDANT_VBLIRQ_THRESH_NS) {
    > The condition should be satisfied if everything works correctly, but also if
    > timestamps would be grossly wrong, thereby leading to a larger than 1 msec
    > positive or negative diff_ns. s64 diff_ns is a signed 64 bit integer. Could
    > abs(diff_ns) somehow miscompute for large 64 bit numbers?
    > All guesswork, the syslog output should tell us more if the timestamping is
    > really involved in the problem.
    > thanks,
    > -mario
    > *********************************************************************
    > Mario Kleiner
    > Max Planck Institute for Biological Cybernetics
    > Spemannstr. 38
    > 72076 Tuebingen
    > Germany
    > e-mail:
    > office: +49 (0)7071/601-1623
    > fax:    +49 (0)7071/601-616
    > www:
    > *********************************************************************
    > "For a successful technology, reality must take precedence
    > over public relations, for Nature cannot be fooled."
    > (Richard Feynman)
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