Quoting Chris Wilson (2019-08-15 20:03:13)
> Quoting Daniel Vetter (2019-08-15 19:48:42)
> > On Thu, Aug 15, 2019 at 8:46 PM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> > > Quoting Daniel Vetter (2019-08-14 18:20:53)
> > > > On Sun, Aug 11, 2019 at 10:15:23AM +0100, Chris Wilson wrote:
> > > > > Now that dma_fence_signal always takes the spinlock to flush the
> > > > > cb_list, simply take the spinlock and call dma_fence_signal_locked() to
> > > > > avoid code repetition.
> > > > >
> > > > > Suggested-by: Christian König <christian.koenig@xxxxxxx>
> > > > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > > > > Cc: Christian König <christian.koenig@xxxxxxx>
> > > >
> > > > Hm, I think this largely defeats the point of having the lockless signal
> > > > enabling trickery in dma_fence. Maybe that part isn't needed by anyone,
> > > > but feels like a thing that needs a notch more thought. And if we need it,
> > > > maybe a bit more cleanup.
> > >
> > > You mean dma_fence_enable_sw_signaling(). The only user appears to be to
> > > flush fences, which is actually the intent of always notifying the signal
> > > cb. By always doing the callbacks, we can avoid installing the interrupt
> > > and completely saturating CPUs with irqs, instead doing a batch in a
> > > leisurely timer callback if not flushed naturally.
> >
> > Yeah I'm not against ditching this,
>
> I was just thinking aloud working out what the current use case in ttm
> was for.
>
> > but can't we ditch a lot more if
> > we just always take the spinlock in those paths now anyways? Kinda not
> > worth having the complexity anymore.
>
> You would be able to drop the was_set from dma_fence_add_callback. Say
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 59ac96ec7ba8..e566445134a2 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -345,38 +345,31 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
> dma_fence_func_t func)
> {
> unsigned long flags;
> - int ret = 0;
> - bool was_set;
> + int ret = -ENOENT;
>
> if (WARN_ON(!fence || !func))
> return -EINVAL;
>
> - if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
> - INIT_LIST_HEAD(&cb->node);
> + INIT_LIST_HEAD(&cb->node);
> + cb->func = func;
> +
> + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> return -ENOENT;
> - }
>
> spin_lock_irqsave(fence->lock, flags);
> -
> - was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> - &fence->flags);
> -
> - if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> - ret = -ENOENT;
> - else if (!was_set && fence->ops->enable_signaling) {
> + if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
> + !test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> + &fence->flags)) {
> trace_dma_fence_enable_signal(fence);
>
> - if (!fence->ops->enable_signaling(fence)) {
> + if (!fence->ops->enable_signaling ||
> + fence->ops->enable_signaling(fence)) {
> + list_add_tail(&cb->node, &fence->cb_list);
> + ret = 0;
> + } else {
> dma_fence_signal_locked(fence);
> - ret = -ENOENT;
> }
> }
> -
> - if (!ret) {
> - cb->func = func;
> - list_add_tail(&cb->node, &fence->cb_list);
> - } else
> - INIT_LIST_HEAD(&cb->node);
> spin_unlock_irqrestore(fence->lock, flags);
>
> return ret;
>
> Not a whole lot changes in terms of branches and serialising
> instructions. One less baffling sequence to worry about.
Fwiw,
Function old new delta
dma_fence_add_callback 338 302 -36
Almost certainly more shaving if you stare.
-Chris
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