From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-wr0-f171.google.com ([209.85.128.171]:41711 "EHLO mail-wr0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751107AbeEDNRM (ORCPT ); Fri, 4 May 2018 09:17:12 -0400 Received: by mail-wr0-f171.google.com with SMTP id g21-v6so21044126wrb.8 for ; Fri, 04 May 2018 06:17:11 -0700 (PDT) Reply-To: christian.koenig@amd.com Subject: Re: [PATCH 04/15] dma-fence: Make ->wait callback optional To: Daniel Vetter , Chris Wilson Cc: Intel Graphics Development , DRI Development , "moderated list:DMA BUFFER SHARING FRAMEWORK" , "open list:DMA BUFFER SHARING FRAMEWORK" References: <20180503142603.28513-1-daniel.vetter@ffwll.ch> <20180503142603.28513-5-daniel.vetter@ffwll.ch> <152542135089.4767.3315686184618150713@mail.alporthouse.com> <20180504081722.GQ12521@phenom.ffwll.local> <20180504082301.GR12521@phenom.ffwll.local> <152542269311.4767.4254637128660397977@mail.alporthouse.com> <20180504085759.GT12521@phenom.ffwll.local> <152542538170.4767.9925437389288286145@mail.alporthouse.com> From: =?UTF-8?Q?Christian_K=c3=b6nig?= Message-ID: <82509ba9-b305-433f-b70c-16ae857d13bc@gmail.com> Date: Fri, 4 May 2018 15:17:08 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-media-owner@vger.kernel.org List-ID: Am 04.05.2018 um 11:25 schrieb Daniel Vetter: > On Fri, May 4, 2018 at 11:16 AM, Chris Wilson 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. Christian. > -Daniel