Re: [PATCH 01/18] drm: Introduce drm_mm_create_block()

Chris Wilson
2012-10-28T10:08:01Z

On Fri, 26 Oct 2012 14:47:43 -0700, Ben Widawsky <ben@bwidawsk.net> wrote:
> On Fri, 19 Oct 2012 18:03:07 +0100
> Chris Wilson <chris@chris-wilson.co.uk> wrote:
>
> > To be used later by i915 to preallocate exact blocks of space from the
> > range manager.
> >
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Dave Airlie <airlied@redhat.com>
> > Cc: dri-devel@lists.freedestkop.org
>
> With bikesheds below addressed or not:
> Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
> > ---
> > drivers/gpu/drm/drm_mm.c | 49
> > ++++++++++++++++++++++++++++++++++++++++++++++
> > include/drm/drm_mm.h | 4 ++++ 2 files changed, 53 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c
> > index 9bb82f7..5db8c20 100644
> > --- a/drivers/gpu/drm/drm_mm.c
> > +++ b/drivers/gpu/drm/drm_mm.c
> > @@ -161,6 +161,55 @@ static void drm_mm_insert_helper(struct
> > drm_mm_node *hole_node, }
> > }
> >
> > +struct drm_mm_node *drm_mm_create_block(struct drm_mm *mm,
> > + unsigned long start,
> > + unsigned long size,
> > + bool atomic)
> > +{
>
> <bikeshed>
> You could add a best_fit field like some of the other interfaces which
> will try to find a start == hole_start and end == hole_end. I'd guess
> this interface won't be called enough to worry about fragmentation too
> much though.
> </bikeshed>

It's not a best fit though, it's an exact allocation request. The search
is to find the location in the free list where we need to insert the
node, and just as importantly reject the request if it would clobber an
earlier allocation.

>
> > + struct drm_mm_node *hole, *node;
> > + unsigned long end = start + size;
> > +
> > + list_for_each_entry(hole, &mm->hole_stack, hole_stack) {
> > + unsigned long hole_start;
> > + unsigned long hole_end;
> > +
> > + BUG_ON(!hole->hole_follows);
>
> <bikeshed>
> This isn't bad, but I don't think sticking the bug here is all that
> helpful in finding where the bug occured, since it wasn't here. WARN is
> perhaps more useful, but equally unhelpful IMO.
> </bikeshed>

The BUG_ON() is to be consistent with the rest of the code, and so there
isn't a conflict of interests when replacing all the common chunks with
drm_mm_for_each_hole().
-Chris

--
Chris Wilson, Intel Open Source Technology Centre