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    SubjectRe: [PATCH] drm/i915: Suppress spurious vblank interrupts
    On Tue, 1 Feb 2011 10:46:17 -0800
    Hugh Dickins <hughd@google.com> wrote:

    > On Tue, Feb 1, 2011 at 10:08 AM, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
    > > On Tue, 1 Feb 2011 09:46:43 -0800
    > > Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
    > >>
    > >> Are you still seeing underruns during normal activity?
    >
    > Yes. That is, I see the "pipe a underrun" messages when I set drm
    > debug 7: I'm unaware of any ill-effect from them, unless they are
    > indeed a factor in my unflushed text issue.

    They can cause your display to shake or flicker, or the memory
    controller to get confused enough to hang your system.

    > I was surprised that i915_driver_irq_handler "Clear the PIPE(A|B)STAT
    > regs" writes back precisely the pipea_stats it reads in, I'd have
    > expected to clear something there (and did earlier experiment with
    > writing back 0: black screen at boot!). But assumed the protocol is
    > such that it acknowledges the status bits by writing same back.

    Exactly. High bits are the interrupt enable bits, low bits are set for
    status, write of 1 to clear status.

    > Right. I haven't double-checked the logic, but I believe it's because
    > of bits set in the underrunning pipea_stats. I did one time modify
    > the underrun message to print out pipea_stats, over five seconds most
    > (265) values were 0x80440207
    > (there were also 14 occurrences of 0x80440007, 5 of 0x80440004 and 3
    > of 0x80440204).

    Well, vblank interrupts are enabled (though they shouldn't be for your
    config as far as we know), so the interrupt status and handling is
    occurring as expected.

    What I find strange is that you're seeing flip pending interrupts. Are
    your symptoms affected if you remove the
    I915_DISPLAY_PLANE_[AB]_FLIP_PENDING_INTERRUPT lines from
    I915_INTERRUPT_ENABLE_FIX at the top of i915_irq.c?

    Do you see any calls to drm_mode_page_flip_ioctl() in your environment?

    > I just tried i915.powersave=0 but the underruns still appeared. I
    > then tried earlier kernels, and was surprised to find no underruns
    > with 2.6.34, 2.6.36: the underruns appeared with 2.6.37.

    Ok, that rules out self-refresh then I think.

    --
    Jesse Barnes, Intel Open Source Technology Center


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