Hi Daniel,
2016-01-19 Daniel Vetter <daniel@xxxxxxxx>:
> On Fri, Jan 15, 2016 at 12:55:10PM -0200, Gustavo Padovan wrote:
> > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> >
> > This patch series de-stage the sync framework, and in order to accomplish that
> > a bunch of cleanups/improvements on the sync and fence were made.
> >
> > The sync framework contained some abstractions around struct fence and those
> > were removed in the de-staging process among other changes:
> >
> > Userspace visible changes
> > -------------------------
> >
> > * The sw_sync file was moved from /dev/sw_sync to <debugfs>/sync/sw_sync. No
> > other change.
> >
> > Kernel API changes
> > ------------------
> >
> > * struct sync_timeline is now struct fence_timeline
> > * sync_timeline_ops is now fence_timeline_ops and they now carry struct
> > fence as parameter instead of struct sync_pt
> > * a .cleanup() fence op was added to allow sync_fence to run a cleanup when
> > the fence_timeline is destroyed
> > * added fence_add_used_data() to pass a private point to struct fence. This
> > pointer is sent back on the .cleanup op.
> > * The sync timeline function were moved to be fence_timeline functions:
> > - sync_timeline_create() -> fence_timeline_create()
> > - sync_timeline_get() -> fence_timeline_get()
> > - sync_timeline_put() -> fence_timeline_put()
> > - sync_timeline_destroy() -> fence_timeline_destroy()
> > - sync_timeline_signal() -> fence_timeline_signal()
> >
> > * sync_pt_create() was replaced be fence_create_on_timeline()
> >
> > Internal changes
> > ----------------
> >
> > * fence_timeline_ops was removed in favor of direct use fence_ops
> > * fence default functions were created for fence_ops
> > * removed structs sync_pt, sw_sync_timeline and sw_sync_pt
>
> Bunch of fairly random comments all over:
>
> - include/uapi/linux/sw_sync.h imo should be dropped, it's just a private
> debugfs interface between fence fds and the testsuite. Since the plan is
> to have the testcases integrated into the kernel tree too we don't need
> a public header.
>
> - similar for include/linux/sw_sync.h Imo that should all be moved into
> sync_debug.c. Same for sw_sync.c, that should all land in sync_debug
> imo, and made optional with a Kconfig option. At least we should reuse
> CONFIG_DEBUGFS.
These two items sounds reasonable to me.
>
> - fence_context and fence_timeline are really the same. timeline has some
> super-basic support for doing sw-only fence timelines, but imo that's
> not really worth keeping (and if so better to keep seperate in a
> sw-fence.c or similar, like seqno-fence.c). The other main thing
> timeline provides is support to clean up fences on a timeline. And imo
> that cleanup should be done by the core fence support, not by the add-on
> stuff.
Yes, they are. But I currently doesn't know how to merge them best, so I
decided to go for a RFC instead of trying some crazy solution touching
all fence_context users.
>
> Interlude about fence cleanup on driver unload:
>
> Working drivers imo should never call timeline_destroy when there's still
> an unsignalled fence around for that timeline/context. That just means
> they're broken and failed to clean up all the pending work. So the problem
> really is only what to do with fences where the driver disappeared, and
> for that we essentially need a fence_revoke() function (which could be
> called internally from timeline_free). So here's what I think
> timeline_free should do:
>
> for_each_fence_on_timel() {
> WARN_ON(!fence_is_signalled());
>
> fence_revoke(fence);
> }
>
> Implementing fence_revoke is a bit tricky since we need to make sure the
> memory contained ->ops and similar stuff doesn't disappear. Simplest
> option might be to grab a temporary reference (using
> kref_get_unless_zero), and then exchange ->ops with one that has only a
> release function. We don't need anything else as long as all fence_*
> functions the kernel might call check for signalling correctly first
> (fence_wait is broken at least).
>
> Or we just give up (for now) and declare module unload as slightly racy.
> dma-buf is similar. An intermediate option might be to at least add a
> THIS_MODULE reference to each fence (but that's a bit expensive ...).
I'd say we just give up for now as we don't have any driver using
timeline_destroy for now. So we could go for other improvements first.
> - back to timeline vs. context: I have no idea how to best clean up this
> mess, but least painful option long-term is probably to switch over all
> current users of fence_context_alloc to timelines and remove the plain
> context interface.
Agreed.
>
> - Imo the interface in include/linux/sync.h is duplicating too much of
> fence.h. I think the only bits we need are the refcounting, creating,
> fd-install and that's it. Plus a macro to loop over all the fences in a
> sync_fence. With that drivers will only ever deal with a pile of
> struct fence, making implicit fencing (using the fence list in dma-buf)
> and explicit fencing (using the fence list in sync_fence) much more
> similar.
Yes, most of the sync_fence waiting should not be exported. Drivers
should only wait for fence imo, not sync_fences.
>
> And we can easily do that since no internal users ;-)
>
> - get_timeline_name and get_driver_name are imo too much indirection, just
> add ->(drv_)name field to each of these.
>
> - struct sync_fence is a major confusion imo against struct fence. It
> made much more sense in the pure-android world where fence == sync_pt.
> Maybe we can rename sync_fence to sync_fence_fd (a bit long, and fd is a
> bit inaccurate), sync_file (like this best), fence_file (sounds silly
> imo), or something else?
sync_file sounds good for me. fence_file feels like it a file for a
single fence but we may have many fences on one sync_file.
>
> - I guess just not yet part of this rfc, but moving the testsuite and
> adding kerneldoc for this is planned I guess? If you feel like I think
> it'd be best. We pull the current dma-buf stuff into
> device-drivers.tmpl, but it's completely lacking overview docs and all
> that. And I'd like to duplicate at least the dma-buf/fence sections into
> the gpu.tmpl docbook.
We have converted testsuite from android's libsync but we need to wait
for Google to re-license it to send it upstream.
kerneldoc is planned for sure, but I'd say it will be better to have
some users first, DRM for example.
>
> - If we make timelines first class objects I think we could move some of
> the fields from struct fence to struct fence_timeline. E.g. the ops
> struct. That also makes it clearer that some of the vfuncs really should
> be taking a struct fence_timeline *timeline instead of a struct fence
> *fence as their primary parameter.
I'll keep that as a final goal and work RFC v2 and see how far we can
get.
Gustavo
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