[RFC PATCH] drm: rework flip-work helpers to avoid calling func when the FIFO is full

[Boris BREZILLON]

On Fri, 11 Jul 2014 11:41:12 -0400
Rob Clark <robdclark at gmail.com> wrote:

> On Fri, Jul 11, 2014 at 11:17 AM, Boris BREZILLON
> <boris.brezillon at free-electrons.com> wrote:
> > Make use of lists instead of kfifo in order to dynamically allocate
> > task entry when someone require some delayed work, and thus preventing
> > drm_flip_work_queue from directly calling func instead of queuing this
> > call.
> > This allow drm_flip_work_queue to be safely called even within irq
> > handlers.
> >
> > Add new helper functions to allocate a flip work task and queue it when
> > needed. This prevents allocating data within irq context (which might
> > impact the time spent in the irq handler).
> >
> > Signed-off-by: Boris BREZILLON <boris.brezillon at free-electrons.com>
> > ---
> > Hi Rob,
> >
> > This is a proposal for what you suggested (dynamically growing the drm
> > flip work queue in order to avoid direct call of work->func when calling
> > drm_flip_work_queue).
> >
> > I'm not sure this is exactly what you expected, because I'm now using
> > lists instead of kfifo (and thus lose the lockless part), but at least
> > we can now safely call drm_flip_work_queue or drm_flip_work_queue_task
> > from irq handlers :-).
> >
> > You were also worried about queueing the same framebuffer multiple times
> > and with this implementation you shouldn't have any problem (at least with
> > drm_flip_work_queue, what people do with drm_flip_work_queue_task is their
> > own responsability, but they should allocate one task for each operation
> > even if they are manipulating the same framebuffer).
> 
> yeah, if we are dynamically allocating the list nodes, that solves the
> queuing-up-multiple-times issue..
> 
> I wonder if drm_flip_work_allocate_task() should use GPF_ATOMIC when
> allocating?

That's funny, I was actually modifying the API to pass gfp_t flags to
this function ;-)

> I guess maybe it is possible to pre-allocate the task
> from non-irq context, and then queue it from irq context.. it makes
> the API a bit more complex, but there are only a couple users
> currently, so I suppose this should be doable.

I tried to keep the existing API so that existing users won't see the
difference (I guess none of them are calling drm_flip_work_queue).

I just added the drm_flip_work_allocate_task and
drm_flip_work_queue_task for those who want more control on the
queuing process.

Best Regards,

Boris





-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com