Am 09.02.23 um 14:06 schrieb Maíra Canal:
On 2/9/23 09:13, Christian König wrote:
Am 09.02.23 um 12:23 schrieb Maíra Canal:
On 2/9/23 05:18, Christian König wrote:
Hello everyone,
the drm_debugfs has a couple of well known design problems.
Especially it wasn't possible to add files between initializing and
registering
of DRM devices since the underlying debugfs directory wasn't
created yet.
The resulting necessity of the driver->debugfs_init() callback
function is a
mid-layering which is really frowned on since it creates a horrible
driver->DRM->driver design layering.
The recent patch "drm/debugfs: create device-centered debugfs
functions" tried
to address those problem, but doesn't seem to work correctly. This
looks like
a misunderstanding of the call flow around drm_debugfs_init(),
which is called
multiple times, once for the primary and once for the render node.
So what happens now is the following:
1. drm_dev_init() initially allocates the drm_minor objects.
2. ... back to the driver ...
3. drm_dev_register() is called.
4. drm_debugfs_init() is called for the primary node.
5. drm_framebuffer_debugfs_init(), drm_client_debugfs_init() and
drm_atomic_debugfs_init() call drm_debugfs_add_file(s)() to add
the files
for the primary node.
6. The driver->debugfs_init() callback is called to add debugfs
files for the
primary node.
7. The added files are consumed and added to the primary node
debugfs directory.
8. drm_debugfs_init() is called for the render node.
9. drm_framebuffer_debugfs_init(), drm_client_debugfs_init() and
drm_atomic_debugfs_init() call drm_debugfs_add_file(s)() to add
the files
again for the render node.
10. The driver->debugfs_init() callback is called to add debugfs
files for the
render node.
11. The added files are consumed and added to the render node
debugfs directory.
12. Some more files are added through drm_debugfs_add_file().
13. drm_debugfs_late_register() add the files once more to the
primary node
debugfs directory.
14. From this point on files added through drm_debugfs_add_file()
are simply ignored.
15. ... back to the driver ...
Because of this the dev->debugfs_mutex lock is also completely
pointless since
any concurrent use of the interface would just randomly either add
the files to
the primary or render node or just not at all.
Even worse is that this implementation nails the coffin for
removing the
driver->debugfs_init() mid-layering because otherwise drivers can't
control
where their debugfs (primary/render node) are actually added.
This patch set here now tries to clean this up a bit, but most
likely isn't
fully complete either since I didn't audit every driver/call path.
I tested the patchset on the v3d, vc4 and vkms and all the files are
generated
as expected, but I'm getting the following errors on dmesg:
[ 3.872026] debugfs: File 'v3d_ident' in directory '0' already
present!
[ 3.872064] debugfs: File 'v3d_ident' in directory '128' already
present!
[ 3.872078] debugfs: File 'v3d_regs' in directory '0' already
present!
[ 3.872087] debugfs: File 'v3d_regs' in directory '128' already
present!
[ 3.872097] debugfs: File 'measure_clock' in directory '0'
already present!
[ 3.872105] debugfs: File 'measure_clock' in directory '128'
already present!
[ 3.872116] debugfs: File 'bo_stats' in directory '0' already
present!
[ 3.872124] debugfs: File 'bo_stats' in directory '128' already
present!
It looks like the render node is being added twice, since this
doesn't happen
for vc4 and vkms.
Thanks for the feedback and yes that's exactly what I meant with that
I haven't looked into all code paths.
Could it be that v3d registers it's debugfs files from the
debugfs_init callback?
Although this is true, I'm not sure if this is the reason why the
files are
being registered twice, as this doesn't happen to vc4, and it also
uses the
debugfs_init callback. I believe it is somewhat related to the fact that
v3d is the primary node and the render node.
I see. Thanks for the hint.
Best Regards,
- Maíra Canal
One alternative would be to just completely nuke support for separate
render node debugfs files and only add a symlink to the primary node.
Opinions?
What do you think of this approach? I can't come up with any reason why
we should have separate debugfs files for render nodes and I think it is
pretty much the same reason you came up with the patch for per device
debugfs files instead of per minor.
Regards,
Christian.
Regards,
Christian.
Otherwise, the patchset looks good to me, but maybe Daniel has some
other
thoughts about it.
Best Regards,
- Maíra Canal
Please comment/discuss.
Cheers,
Christian.
