[PATCH 3/3] drm/debugfs: remove dev->debugfs_list and debugfs_mutex
Daniel Vetter
daniel at ffwll.ch
Fri Feb 17 19:38:28 UTC 2023
On Fri, Feb 17, 2023 at 11:01:18AM +0100, Stanislaw Gruszka wrote:
> On Fri, Feb 17, 2023 at 10:22:25AM +0100, Christian König wrote:
> > Am 16.02.23 um 20:54 schrieb Daniel Vetter:
> > > On Thu, Feb 16, 2023 at 07:08:49PM +0200, Jani Nikula wrote:
> > > > On Thu, 16 Feb 2023, Christian König <christian.koenig at amd.com> wrote:
> > > > > Am 16.02.23 um 17:46 schrieb Jani Nikula:
> > > > > > On Thu, 16 Feb 2023, Christian König <christian.koenig at amd.com> wrote:
> > > > > > > Am 16.02.23 um 12:33 schrieb Daniel Vetter:
> > > > > > > > On Thu, Feb 09, 2023 at 09:18:38AM +0100, Christian König wrote:
> > > > > > > > > The mutex was completely pointless in the first place since any
> > > > > > > > > parallel adding of files to this list would result in random
> > > > > > > > > behavior since the list is filled and consumed multiple times.
> > > > > > > > >
> > > > > > > > > Completely drop that approach and just create the files directly.
> > > > > > > > >
> > > > > > > > > This also re-adds the debugfs files to the render node directory and
> > > > > > > > > removes drm_debugfs_late_register().
> > > > > > > > >
> > > > > > > > > Signed-off-by: Christian König <christian.koenig at amd.com>
> > > > > > > > > ---
> > > > > > > > > drivers/gpu/drm/drm_debugfs.c | 32 +++++++------------------------
> > > > > > > > > drivers/gpu/drm/drm_drv.c | 3 ---
> > > > > > > > > drivers/gpu/drm/drm_internal.h | 5 -----
> > > > > > > > > drivers/gpu/drm/drm_mode_config.c | 2 --
> > > > > > > > > include/drm/drm_device.h | 15 ---------------
> > > > > > > > > 5 files changed, 7 insertions(+), 50 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
> > > > > > > > > index 558e3a7271a5..a40288e67264 100644
> > > > > > > > > --- a/drivers/gpu/drm/drm_debugfs.c
> > > > > > > > > +++ b/drivers/gpu/drm/drm_debugfs.c
> > > > > > > > > @@ -246,31 +246,9 @@ void drm_debugfs_dev_register(struct drm_device *dev)
> > > > > > > > > void drm_debugfs_minor_register(struct drm_minor *minor)
> > > > > > > > > {
> > > > > > > > > struct drm_device *dev = minor->dev;
> > > > > > > > > - struct drm_debugfs_entry *entry, *tmp;
> > > > > > > > > if (dev->driver->debugfs_init)
> > > > > > > > > dev->driver->debugfs_init(minor);
> > > > > > > > > -
> > > > > > > > > - list_for_each_entry_safe(entry, tmp, &dev->debugfs_list, list) {
> > > > > > > > > - debugfs_create_file(entry->file.name, 0444,
> > > > > > > > > - minor->debugfs_root, entry, &drm_debugfs_entry_fops);
> > > > > > > > > - list_del(&entry->list);
> > > > > > > > > - }
> > > > > > > > > -}
> > > > > > > > > -
> > > > > > > > > -void drm_debugfs_late_register(struct drm_device *dev)
> > > > > > > > > -{
> > > > > > > > > - struct drm_minor *minor = dev->primary;
> > > > > > > > > - struct drm_debugfs_entry *entry, *tmp;
> > > > > > > > > -
> > > > > > > > > - if (!minor)
> > > > > > > > > - return;
> > > > > > > > > -
> > > > > > > > > - list_for_each_entry_safe(entry, tmp, &dev->debugfs_list, list) {
> > > > > > > > > - debugfs_create_file(entry->file.name, 0444,
> > > > > > > > > - minor->debugfs_root, entry, &drm_debugfs_entry_fops);
> > > > > > > > > - list_del(&entry->list);
> > > > > > > > > - }
> > > > > > > > > }
> > > > > > > > > int drm_debugfs_remove_files(const struct drm_info_list *files, int count,
> > > > > > > > > @@ -343,9 +321,13 @@ void drm_debugfs_add_file(struct drm_device *dev, const char *name,
> > > > > > > > > entry->file.data = data;
> > > > > > > > > entry->dev = dev;
> > > > > > > > > - mutex_lock(&dev->debugfs_mutex);
> > > > > > > > > - list_add(&entry->list, &dev->debugfs_list);
> > > > > > > > > - mutex_unlock(&dev->debugfs_mutex);
> > > > > > > > > + debugfs_create_file(name, 0444, dev->primary->debugfs_root, entry,
> > > > > > > > > + &drm_debugfs_entry_fops);
> > > > > > > > > +
> > > > > > > > > + /* TODO: This should probably only be a symlink */
> > > > > > > > > + if (dev->render)
> > > > > > > > > + debugfs_create_file(name, 0444, dev->render->debugfs_root,
> > > > > > > > > + entry, &drm_debugfs_entry_fops);
> > > > > > > > Nope. You are fundamentally missing the point of all this, which is:
> > > > > > > >
> > > > > > > > - drivers create debugfs files whenever they want to, as long as it's
> > > > > > > > _before_ drm_dev_register is called.
> > > > > > > >
> > > > > > > > - drm_dev_register will set them all up.
> > > > > > > >
> > > > > > > > This is necessary because otherwise you have the potential for some nice
> > > > > > > > oops and stuff when userspace tries to access these files before the
> > > > > > > > driver is ready.
> > > > > > > >
> > > > > > > > Note that with sysfs all this infrastructure already exists, which is why
> > > > > > > > you can create sysfs files whenever you feel like, and things wont go
> > > > > > > > boom.
> > > > > > > Well Yeah I've considered that, I just don't think it's a good idea for
> > > > > > > debugfs.
> > > > > > >
> > > > > > > debugfs is meant to be a helper for debugging things and that especially
> > > > > > > includes the time between drm_dev_init() and drm_dev_register() because
> > > > > > > that's where we probe the hardware and try to get it working.
> > > > > > >
> > > > > > > Not having the debugfs files which allows for things like hardware
> > > > > > > register access and reading internal state during that is a really and I
> > > > > > > mean REALLY bad idea. This is essentially what we have those files for.
> > > > > > So you mean you want to have early debugfs so you can have some script
> > > > > > hammering the debugfs to get info out between init and register during
> > > > > > probe?
> > > > > Well not hammering. What we usually do in bringup is to set firmware
> > > > > timeout to infinity and the driver then sits and waits for the hw.
> > > > >
> > > > > The tool used to access registers then goes directly through the PCI bar
> > > > > at the moment, but that's essentially a bad idea for registers which you
> > > > > grab a lock for to access (like index/data).
> > > > >
> > > > > > I just think registering debugfs before everything is ready is a recipe
> > > > > > for disaster. All of the debugfs needs to check all the conditions that
> > > > > > they need across all of the probe stages. It'll be difficult to get it
> > > > > > right. And you'll get cargo culted checks copy pasted all over the
> > > > > > place.
> > > > > Yeah, but it's debugfs. That is not supposed to work under all conditions.
> > > > >
> > > > > Just try to read amdgpu_regs on a not existing register index. This will
> > > > > just hang or reboot your box immediately on APUs.
> > > > I'm firmly in the camp that debugfs does not need to work under all
> > > > conditions, but that it must fail gracefully instead of crashing.
> > > Yeah I mean once we talk bring-up, you can just hand-roll the necessary
> > > bring debugfs things that you need to work before the driver is ready to
> > > do anything.
> > >
> > > But bring-up debugfs fun is rather special, same way pre-silicon support
> > > tends to be rather special. Shipping that in distros does not sound like a
> > > good idea at all to me.
> >
> > Yeah, that's indeed a really good point.
> >
> > I can't remember how often I had to note that module parameters would also
> > be used by end users.
> >
> > How about if the create the debugfs directory with a "." as name prefix
> > first and then rename it as soon as the device is registered?
>
> Good idea. Or the dir could have this drm_dev->unique name and be created
> during alloc, and link in minor created during registration. That would
> mean minor link is safe to use and unique potentially dangerous before
> registration.
>
> > Alternatively
> > we could clear the i_mode of the directory.
>
> I checked that yesterday and this does not prevent to access the file
> for root user. Perhaps there is other smart way for blocking
> root access in vfs just by modifying some inode field, but just
> 'chmod 0000 file' does not prevent that.
>
> > If a power user or engineer wants to debug startup problems stuff it should
> > be trivial to work around that from userspace, and if people do such things
> > they should also know the potential consequences.
>
> Fully agree.
So what about a drm module option instead (that taints the kernel as usual
for these), which:
- registers the debugfs dir right away
- registers any debugfs files as soon as they get populated, instead of
postponing until drm_dev_register
It would only neatly work with the add_file stuff, but I guess drivers
could still hand-roll this if needed.
I think funny games with trying to hide the files while not hiding them is
not a great idea, and explicit "I'm debugging stuff, please stand back"
knob sounds much better to me.
-Daniel
>
> Regards
> Stanislaw
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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