Re: [RFC PATCH 00/12] Ion cleanup in preparation for moving out of staging

[Rob Clark]

On Fri, Mar 10, 2017 at 7:40 AM, Daniel Vetter <daniel@xxxxxxxx> wrote:
> On Fri, Mar 10, 2017 at 10:31:13AM +0000, Brian Starkey wrote:
>> Hi,
>>
>> On Thu, Mar 09, 2017 at 09:38:49AM -0800, Laura Abbott wrote:
>> > On 03/09/2017 02:00 AM, Benjamin Gaignard wrote:
>>
>> [snip]
>>
>> > >
>> > > For me those patches are going in the right direction.
>> > >
>> > > I still have few questions:
>> > > - since alignment management has been remove from ion-core, should it
>> > > be also removed from ioctl structure ?
>> >
>> > Yes, I think I'm going to go with the suggestion to fixup the ABI
>> > so we don't need the compat layer and as part of that I'm also
>> > dropping the align argument.
>> >
>>
>> Is the only motivation for removing the alignment parameter that
>> no-one got around to using it for something useful yet?
>> The original comment was true - different devices do have different
>> alignment requirements.
>>
>> Better alignment can help SMMUs use larger blocks when mapping,
>> reducing TLB pressure and the chance of a page table walk causing
>> display underruns.
>
> Extending ioctl uapi is easy, trying to get rid of bad uapi is much
> harder. Given that right now we don't have an ion allocator that does
> alignment I think removing it makes sense. And if we go with lots of
> heaps, we might as well have an ion heap per alignment that your hw needs,
> so there's different ways to implement this in the future.

slight correction:  if you plan ahead (and do things like zero init if
userspace passes in a smaller ioctl struct like drm_ioctl does),
extending ioctl uapi is easy.. might be something worth fixing from
the get-go..

BR,
-R

> At least from the unix device memory allocator pov it's probably simpler
> to encode stuff like this into the heap name, instead of having to pass
> heap + list of additional properties/constraints.
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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