Am 09.02.23 um 15:19 schrieb Maxime Ripard:
On Thu, Feb 09, 2023 at 03:06:10PM +0100, Christian König wrote:
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.
They are two entirely separate devices and drivers, it doesn't make much
sense to move their debugfs files to one or the other.
Well exactly that isn't true. The primary and render node are just two
file under /dev for the same hardware device and driver.
We just offer different functionality through the two interfaces, but
essentially there isn't any information we could expose for one which
isn't true for the other as well.
Christian.
Maxime
