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

[Christian König]

Am 02.07.2018 um 10:23 schrieb Daniel Vetter:
> 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 at 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.
> 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 ...

Feel free to add my Acked-by for now, but I still have a kind of a gut 
feeling that we might want to revisit this decision at some time.

Christian.

>
> I went ahead and applied the first three patches of this series meanwhile.
> -Daniel