Re: [PATCH 29/30] drm/i915: Track fence setup separately from fenced object lifetime

From: Daniel Vetter <daniel@ffwll.ch>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andy Whitcroft <apw@canonical.com>, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 29/30] drm/i915: Track fence setup separately from fenced object lifetime
Date: Wed, 13 Apr 2011 22:42:23 +0200	[thread overview]
Message-ID: <20110413204222.GK3660@viiv.ffwll.ch> (raw)
In-Reply-To: <1302640318-23165-30-git-send-email-chris@chris-wilson.co.uk>

On Tue, Apr 12, 2011 at 09:31:57PM +0100, Chris Wilson wrote:
> This fixes a bookkeeping error causing an OOPS whilst waiting for an
> object to finish using a fence. Now we can simply wait for the fence to
> be written independent of the objects currently inhabiting it (past,
> present and future).
> 
> A large amount of the change is to delay updating the information about
> the fence on bo until after we successfully write, or queue the write to,
> the register. This avoids the complication of undoing a partial change
> should we fail in pipelining the change.
> 
> Cc: Andy Whitcroft <apw@canonical.com>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

I think that r-b is stale ;-) Still holds though for the general idea. A
few nitpicks below.

On general comment: I think we should get completely rid of
last_fenced_ring. There should be no way an object can change rings
without being at least completely flushed (or even going through the
inactive list).  Maybe that's for a separate patch but I'm slightly uneasy
with the fact that we don't seem to systematically clear last_fenced_ring
_anywhere_.

> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index ca14a86..1949048 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1731,6 +1731,8 @@ i915_gem_object_move_to_inactive(struct drm_i915_gem_object *obj)
>  	i915_gem_object_move_off_active(obj);
>  	obj->fenced_gpu_access = false;
>  
> +	obj->last_fenced_seqno = 0;
> +

I think we could move that to move_off_active where last_rendering_seqno
is being reset. Would be slightly more consistent. Resetting
last_fenced_ring together with last_fenced_seqno probably makes sens, too.

> @@ -2675,47 +2661,43 @@ i915_gem_object_get_fence(struct drm_i915_gem_object *obj,
>  	if (reg == NULL)
>  		return -ENOSPC;
>  
> -	ret = i915_gem_object_flush_fence(obj, pipelined);
> -	if (ret)
> -		return ret;
> -
> -	if (reg->obj) {
> -		struct drm_i915_gem_object *old = reg->obj;
> -
> +	if ((old = reg->obj)) {

Argh. Can you move the assignment out?

> @@ -2732,7 +2714,31 @@ update:
>  		ret = i830_write_fence_reg(obj, pipelined, regnum);
>  		break;
>  	}
> +	if (ret)
> +		goto err;
> +
> +	if (pipelined) {
> +		reg->setup_seqno = i915_gem_next_request_seqno(pipelined);
> +		reg->setup_ring = pipelined;
> +		if (old) {
> +			old->last_fenced_ring = pipelined;
> +			old->last_fenced_seqno = reg->setup_seqno;
> +		}

This looks superfluous. flush_fence should take care of this either
directly or via flush_ring -> process_flushing_list -> move_to_active.
If it's just paranoia, can this be converted to a WARN_ON? Or is this
closing a gap I'm not seeing?
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48