Re: [PATCH v2 01/11] dma-buf/sync_file: de-stage sync_file

From: Daniel Vetter <daniel@ffwll.ch>
To: Greg Hackmann <ghackmann@google.com>
Cc: devel@driverdev.osuosl.org,
	"Daniel Stone" <daniels@collabora.com>,
	"Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
	"ML dri-devel" <dri-devel@lists.freedesktop.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Emil Velikov" <emil.l.velikov@gmail.com>,
	"Arve Hjønnevåg" <arve@android.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Riley Andrews" <riandrews@android.com>,
	"Gustavo Padovan" <gustavo.padovan@collabora.co.uk>,
	"John Harrison" <John.C.Harrison@intel.com>
Subject: Re: [PATCH v2 01/11] dma-buf/sync_file: de-stage sync_file
Date: Thu, 28 Jan 2016 10:23:55 +0100	[thread overview]
Message-ID: <20160128092355.GS11240@phenom.ffwll.local> (raw)
In-Reply-To: <56A9396F.8010803@google.com>

On Wed, Jan 27, 2016 at 01:41:03PM -0800, Greg Hackmann wrote:
> On 01/27/2016 12:25 PM, Gustavo Padovan wrote:
> >>>>Is there a value in keeping the abi unchanged?
> >>>>If not, then Documentation/ioctl/botching-up-ioctls.txt is worth a read.
> >>>
> >>>None from me. I'll look where we can improve the ABI.
> 
> Android has existing clients of the current ABI.  Thankfully they're all
> contained in system services like SurfaceFlinger, since end-user apps don't
> get direct access to fence fds.
> 
> As long the ABI breaks don't remove functionality we depend on, we can wrap
> around them in our userspace libsync.  I'd rather not have to do that, but
> it's a price I'm willing to pay to get this moved out of staging.
> 
> >>  - struct sync_file_info_data::fence_info is of type __u8 yet it is "a
> >>fence_info struct for every fence in the sync_file". Thus shouldn't
> >>one use "struct fence_info" as the type ?
> >
> >Agreed. But I'm currently thinking if we really should keep this ioctl.
> >
> >	Gustavo
> >
> 
> I'm not seeing any consumers of driver_data in our tree.  OTOH completely
> getting rid of the ioctl would be a problem, since SurfaceFlinger depends on
> the timestamp information for its own bookkeeping.

If we remove driver_data (and len is superflous too), then I think we
should also make the master struct use common ioctl pattern:
- Add a num_fences field or similar that the kernel fills out.
- Make pt_info an __u64 pointer instead of a variable-length array (and
  length) - ioctl payload sizes are somewhat limited.

This way the interface is future-proofed for truly patalogical number of
fences (which surface flinger won't do, but could happen in
server/opencl/media workloads I'd imagine).

And I think driver_data really shouldn't be there, it makes things
complicated with the array of variable-sized objects, and generic
userspace can't really use it - for debug output we already have
obj/driver_name per fence point, which I think is good enough.  

Would that be ok for you from the Android side if Gustavo also provides a
patch to update libsync? I don't think the ABI is fundamentally broken,
but this light cleanup would be nice.

Wrt keeping SYNC_WAIT: I think that's totally fine. Redundant since
polling is supported, but not really an issue imo either. If we're totally
lazy we could implement SYNC_WAIT internally using poll and shave off a
few lines of the implementation.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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