On Mon, Feb 04, 2019 at 06:35:28PM +0100, Noralf Trønnes wrote:
>
>
> Den 04.02.2019 16.41, skrev Daniel Vetter:
> > On Sun, Feb 03, 2019 at 04:41:56PM +0100, Noralf Trønnes wrote:
> >> The only thing now that makes drm_dev_unplug() special is that it sets
> >> drm_device->unplugged. Move this code to drm_dev_unregister() so that we
> >> can remove drm_dev_unplug().
> >>
> >> Signed-off-by: Noralf Trønnes <noralf@xxxxxxxxxxx>
> >> ---
>
> [...]
>
> >> drivers/gpu/drm/drm_drv.c | 27 +++++++++++++++------------
> >> include/drm/drm_drv.h | 10 ++++------
> >> 2 files changed, 19 insertions(+), 18 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> >> index 05bbc2b622fc..e0941200edc6 100644
> >> --- a/drivers/gpu/drm/drm_drv.c
> >> +++ b/drivers/gpu/drm/drm_drv.c
> >> @@ -366,15 +366,6 @@ EXPORT_SYMBOL(drm_dev_exit);
> >> */
> >> void drm_dev_unplug(struct drm_device *dev)
> >> {
> >> - /*
> >> - * After synchronizing any critical read section is guaranteed to see
> >> - * the new value of ->unplugged, and any critical section which might
> >> - * still have seen the old value of ->unplugged is guaranteed to have
> >> - * finished.
> >> - */
> >> - dev->unplugged = true;
> >> - synchronize_srcu(&drm_unplug_srcu);
> >> -
> >> drm_dev_unregister(dev);
> >> drm_dev_put(dev);
> >> }
> >> @@ -832,11 +823,14 @@ EXPORT_SYMBOL(drm_dev_register);
> >> * drm_dev_register() but does not deallocate the device. The caller must call
> >> * drm_dev_put() to drop their final reference.
> >> *
> >> - * A special form of unregistering for hotpluggable devices is drm_dev_unplug(),
> >> - * which can be called while there are still open users of @dev.
> >> + * This function can be called while there are still open users of @dev as long
> >> + * as the driver protects its device resources using drm_dev_enter() and
> >> + * drm_dev_exit().
> >> *
> >> * This should be called first in the device teardown code to make sure
> >> - * userspace can't access the device instance any more.
> >> + * userspace can't access the device instance any more. Drivers that support
> >> + * device unplug will probably want to call drm_atomic_helper_shutdown() first
> >
> > Read once more with a bit more coffee, spotted this:
> >
> > s/first/afterwards/ - shutting down the hw before we've taken it away from
> > userspace is kinda the wrong way round. It should be the inverse of driver
> > load, which is 1) allocate structures 2) prep hw 3) register driver with
> > the world (simplified ofc).
> >
>
> The problem is that drm_dev_unregister() sets the device as unplugged
> and if drm_atomic_helper_shutdown() is called afterwards it's not
> allowed to touch hardware.
>
> I know it's the wrong order, but the only way to do it in the right
> order is to have a separate function that sets unplugged:
>
> drm_dev_unregister();
> drm_atomic_helper_shutdown();
> drm_dev_set_unplugged();
Annoying ... but yeah calling _shutdown() before we stopped userspace is
also not going to work. Because userspace could quickly re-enable
something, and then the refcounts would be all wrong again and leaking
objects.
I get a bit the feeling we're over-optimizing here with trying to devm-ize
drm_dev_register. Just getting drm_device correctly devm-ized is a big
step forward already, and will open up a lot of TODO items across a lot of
drivers. E.g. we could add a drm_dev_kzalloc, for allocating all the drm_*
structs, which gets released together with drm_device. I think that's a
much clearer path forward, I think we all agree that getting the kfree out
of the driver codes is a good thing, and it would allow us to do this
correctly.
Then once we have that and rolled out to a few drivers we can reconsider
the entire unregister/shutdown gordian knot here. Atm I just have no idea
how to do this properly :-/
Thoughts, other ideas?
Cheers, Daniel
> Noralf.
>
> >> + * in order to disable the hardware on regular driver module unload.
> >> */
> >> void drm_dev_unregister(struct drm_device *dev)
> >> {
> >> @@ -845,6 +839,15 @@ void drm_dev_unregister(struct drm_device *dev)
> >> if (drm_core_check_feature(dev, DRIVER_LEGACY))
> >> drm_lastclose(dev);
> >>
> >> + /*
> >> + * After synchronizing any critical read section is guaranteed to see
> >> + * the new value of ->unplugged, and any critical section which might
> >> + * still have seen the old value of ->unplugged is guaranteed to have
> >> + * finished.
> >> + */
> >> + dev->unplugged = true;
> >> + synchronize_srcu(&drm_unplug_srcu);
> >> +
> >> dev->registered = false;
> >>
> >> drm_client_dev_unregister(dev);
> >> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
> >> index ca46a45a9cce..c50696c82a42 100644
> >> --- a/include/drm/drm_drv.h
> >> +++ b/include/drm/drm_drv.h
> >> @@ -736,13 +736,11 @@ void drm_dev_unplug(struct drm_device *dev);
> >> * drm_dev_is_unplugged - is a DRM device unplugged
> >> * @dev: DRM device
> >> *
> >> - * This function can be called to check whether a hotpluggable is unplugged.
> >> - * Unplugging itself is singalled through drm_dev_unplug(). If a device is
> >> - * unplugged, these two functions guarantee that any store before calling
> >> - * drm_dev_unplug() is visible to callers of this function after it completes
> >> + * This function can be called to check whether @dev is unregistered. This can
> >> + * be used to detect that the underlying parent device is gone.
> >
> > I think it'd be good to keep the first part, and just update the reference
> > to drm_dev_unregister. So:
> >
> > * This function can be called to check whether a hotpluggable is unplugged.
> > * Unplugging itself is singalled through drm_dev_unregister(). If a device is
> > * unplugged, these two functions guarantee that any store before calling
> > * drm_dev_unregister() is visible to callers of this function after it
> > * completes.
> >
> > I think your version shrugs a few important details under the rug. With
> > those nits addressed:
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> >
> > Cheers, Daniel
> >
> >> *
> >> - * WARNING: This function fundamentally races against drm_dev_unplug(). It is
> >> - * recommended that drivers instead use the underlying drm_dev_enter() and
> >> + * WARNING: This function fundamentally races against drm_dev_unregister(). It
> >> + * is recommended that drivers instead use the underlying drm_dev_enter() and
> >> * drm_dev_exit() function pairs.
> >> */
> >> static inline bool drm_dev_is_unplugged(struct drm_device *dev)
> >> --
> >> 2.20.1
> >>
> >> _______________________________________________
> >> Intel-gfx mailing list
> >> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> >> https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> >
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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