On Sat, Mar 15, 2014 at 12:08:55AM +0100, Daniel Vetter wrote:
> There's an entire pile of issues in here:
>
> - Use the main RING_HEAD register, not ACTHD. ACTHD points at the gtt
> offset of the batch buffer when a batch is executed. Semaphores are
> always emitted to the main ring, so we always want to look at that.
True, nice catch that would explain a hard hang quite neatly if we tried
to read from beyond the aperture.
> - Mask the obtained HEAD pointer with the actual ring size, which is
> much smaller. Together with the above issue this resulted us in
> trying to dereference a pointer way outside of the ring mmio
> mapping. The resulting invalid access in interrupt context
> (hangcheck is executed from timers) lead to a full blown kernel
> panic. The fbcon panic handler then tried to frob our driver harder,
> resulting in a full machine hang at least on my snb here where I've
> stumbled over this.
>
> - Handle ring wrapping correctly and be a bit more explicit about how
> many dwords we're scanning. We probably should also scan more than
> just 4 ...
You can ignore ring wrapping as valid commands cannot wrap, hence the
formulation of acthd_min.
/*
* Be paranoid and presume the hw has gone off into the wild -
* our ring is smaller than what the hardware (and hence
* HEAD_ADDR) allows.
*/
head = I915_READ_HEAD(ring) & (ring->size - 1);
head_min = max((int)head - 3 * 4, 0);
while (head >= head_min) {
cmd = ioread32(ring->virtual_start + head);
if (cmd == ipehr)
break;
head -= 4;
}
> + /*
> + * Be paranoid and presume the hw has gone off into the wild -
> + * our ring is smaller than what the hardware (and hence
> + * HEAD_ADDR) allows. Also handles wrap-around.
> + */
> + head &= ring->size - 1;
> +
> + /* This here seems to blow up */
> + cmd = ioread32(ring->virtual_start + head);
So what does this mean?
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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