On Fri, May 04, 2018 at 03:47:59PM +0200, Daniel Vetter wrote: > 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@xxxxxxxxxxxxxxxxxx> 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. So ack/nack on this (i.e. do you retract your original r-b or not)? It's kinda holding up all the cleanup patches below ... I went ahead and applied the first three patches of this series meanwhile. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
