Re: [PATCH 04/15] dma-fence: Make ->wait callback optional

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, May 04, 2018 at 03:17:08PM +0200, Christian König wrote:
> Am 04.05.2018 um 11:25 schrieb Daniel Vetter:
> > On Fri, May 4, 2018 at 11:16 AM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > > Quoting Daniel Vetter (2018-05-04 09:57:59)
> > > > On Fri, May 04, 2018 at 09:31:33AM +0100, Chris Wilson wrote:
> > > > > Quoting Daniel Vetter (2018-05-04 09:23:01)
> > > > > > On Fri, May 04, 2018 at 10:17:22AM +0200, Daniel Vetter wrote:
> > > > > > > On Fri, May 04, 2018 at 09:09:10AM +0100, Chris Wilson wrote:
> > > > > > > > Quoting Daniel Vetter (2018-05-03 15:25:52)
> > > > > > > > > Almost everyone uses dma_fence_default_wait.
> > > > > > > > > 
> > > > > > > > > v2: Also remove the BUG_ON(!ops->wait) (Chris).
> > > > > > > > I just don't get the rationale for implicit over explicit.
> > > > > > > Closer approximation of dwim semantics. There's been tons of patch series
> > > > > > > all over drm and related places to get there, once we have a big pile of
> > > > > > > implementations and know what the dwim semantics should be. Individually
> > > > > > > they're all not much, in aggregate they substantially simplify simple
> > > > > > > drivers.
> > > > > > I also think clearer separation between optional optimization hooks and
> > > > > > mandatory core parts is useful in itself.
> > > > > A new spelling of midlayer ;) I don't see the contradiction with a
> > > > > driver saying use the default and simplicity. (I know which one the
> > > > > compiler thinks is simpler ;)
> > > > If the compiler overhead is real then I guess it would makes to be
> > > > explicit. I don't expect that to be a problem though for a blocking
> > > > function.
> > > > 
> > > > I disagree on this being a midlayer - you can still overwrite everything
> > > > you please to. What it does help is people doing less copypasting (and
> > > > assorted bugs), at least in the grand scheme of things. And we do have a
> > > > _lot_ more random small drivers than just a few years ago. Reducing the
> > > > amount of explicit typing just to get default bahaviour has been an
> > > > ongoing theme for a few years now, and your objection here is about the
> > > > first that this is not a good idea. So I'm somewhat confused.
> > > I'm just saying I don't see any rationale for this patch.
> > > 
> > >          "Almost everyone uses dma_fence_default_wait."
> > > 
> > > Why change?
> > > 
> > > Making it look simpler on the surface, so that you don't have to think
> > > about things straight away? I understand the appeal, but I do worry
> > > about it just being an illusion. (Cutting and pasting a line saying
> > > .wait = default_wait, doesn't feel that onerous, as you likely cut and
> > > paste the ops anyway, and at the very least you are reminded about some
> > > of the interactions. You could even have default initializers and/or
> > > magic macros to hide the cut and paste; maybe a simple_dma_fence [now
> > > that's a midlayer!] but I haven't looked.)
> > In really monolithic vtables like drm_driver we do use default
> > function macros, so you type 1 line, get them all. But dma_fence_ops
> > is pretty small, and most drivers only implement a few callbacks. Also
> > note that e.g. the ->release callback already works like that, so this
> > pattern is there already. I simply extended it to ->wait and
> > ->enable_signaling. Also note that I leave the EXPORT_SYMBOL in place,
> > you can still wrap dma_fence_default_wait if you wish to do so.
> > 
> > But I just realized that I didn't clean out the optional release
> > hooks, I guess I should do that too (for the few cases it's not yet
> > done) and respin.
> 
> I kind of agree with Chris here, but also see the practical problem to copy
> the default function in all the implementations.
> 
> We had the same problem in TTM and I also don't really like the result to
> always have that "if (some_callback) default(); else some_callback();".
> 
> Might be that the run time overhead is negligible, but it doesn't feels
> right from the coding style perspective.

Hm, maybe I've seen too much bad code, but modeset helpers is choke full
of exactly that pattern. It's imo also a trade-off. If you have a fairly
specialized library like ttm that's used by relatively few things, doing
everything explicitly is probably better. It's also where kms started out
from.

But if you have a huge pile of fairly simple drivers, imo the balance
starts to tip the other way, and a bit of additional logic in the shared
code to make all the implementations a notch simpler is good. If we
wouldn't have acquired quite a pile of dma_fence implementations I
wouldn't have bothered with all this.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch