Re: [PATCH 12/12] drm: make sysfs device always available for minors

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jul 24, 2014 at 12:48:49PM +0200, David Herrmann wrote:
> Hi
> 
> On Thu, Jul 24, 2014 at 12:36 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Wed, Jul 23, 2014 at 05:26:47PM +0200, David Herrmann wrote:
> >> For each minor we allocate a sysfs device as minor->kdev. Currently, this
> >> is allocated and registered in drm_minor_register(). This makes it
> >> impossible to add sysfs-attributes to the device before it is registered.
> >> Therefore, they are not added atomically, nor can we move device_add()
> >> *after* ->load() is called.
> >>
> >> This patch makes minor->kdev available early, but only adds the device
> >> during minor-registration.
> >>
> >> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
> >
> > Some diff got confused with this one. One comment below, but this is
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >
> >> ---
> >>  drivers/gpu/drm/drm_stub.c  | 22 ++++++++---
> >>  drivers/gpu/drm/drm_sysfs.c | 90 +++++++++++++++++++--------------------------
> >>  include/drm/drmP.h          |  3 +-
> >>  3 files changed, 54 insertions(+), 61 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
> >> index 8b24db5..09c6bfb 100644
> >> --- a/drivers/gpu/drm/drm_stub.c
> >> +++ b/drivers/gpu/drm/drm_stub.c
> >> @@ -281,9 +281,19 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
> >>
> >>       minor->index = r;
> >>
> >> +     minor->kdev = drm_sysfs_minor_alloc(minor);
> >> +     if (IS_ERR(minor->kdev)) {
> >> +             r = PTR_ERR(minor->kdev);
> >> +             goto err_index;
> >> +     }
> >> +
> >>       *drm_minor_get_slot(dev, type) = minor;
> >>       return 0;
> >>
> >> +err_index:
> >> +     spin_lock_irqsave(&drm_minor_lock, flags);
> >> +     idr_remove(&drm_minors_idr, minor->index);
> >> +     spin_unlock_irqrestore(&drm_minor_lock, flags);
> >>  err_free:
> >>       kfree(minor);
> >>       return r;
> >> @@ -300,6 +310,7 @@ static void drm_minor_free(struct drm_device *dev, unsigned int type)
> >>               return;
> >>
> >>       drm_mode_group_destroy(&minor->mode_group);
> >> +     put_device(minor->kdev);
> >>
> >>       spin_lock_irqsave(&drm_minor_lock, flags);
> >>       idr_remove(&drm_minors_idr, minor->index);
> >> @@ -327,11 +338,9 @@ static int drm_minor_register(struct drm_device *dev, unsigned int type)
> >>               return ret;
> >>       }
> >>
> >> -     ret = drm_sysfs_device_add(minor);
> >> -     if (ret) {
> >> -             DRM_ERROR("DRM: Error sysfs_device_add.\n");
> >> +     ret = device_add(minor->kdev);
> >> +     if (ret)
> >>               goto err_debugfs;
> >> -     }
> >>
> >>       /* replace NULL with @minor so lookups will succeed from now on */
> >>       spin_lock_irqsave(&drm_minor_lock, flags);
> >> @@ -352,7 +361,7 @@ static void drm_minor_unregister(struct drm_device *dev, unsigned int type)
> >>       unsigned long flags;
> >>
> >>       minor = *drm_minor_get_slot(dev, type);
> >> -     if (!minor || !minor->kdev)
> >> +     if (!minor || !device_is_registered(minor->kdev))
> >>               return;
> >>
> >>       /* replace @minor with NULL so lookups will fail from now on */
> >> @@ -360,8 +369,9 @@ static void drm_minor_unregister(struct drm_device *dev, unsigned int type)
> >>       idr_replace(&drm_minors_idr, NULL, minor->index);
> >>       spin_unlock_irqrestore(&drm_minor_lock, flags);
> >>
> >> +     device_del(minor->kdev);
> >> +     dev_set_drvdata(minor->kdev, NULL); /* safety belt */
> >>       drm_debugfs_cleanup(minor);
> >> -     drm_sysfs_device_remove(minor);
> >>  }
> >>
> >>  /**
> >> diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> >> index 7827dad..ab1a5f6 100644
> >> --- a/drivers/gpu/drm/drm_sysfs.c
> >> +++ b/drivers/gpu/drm/drm_sysfs.c
> >> @@ -493,71 +493,55 @@ static void drm_sysfs_release(struct device *dev)
> >>  }
> >>
> >>  /**
> >> - * drm_sysfs_device_add - adds a class device to sysfs for a character driver
> >> - * @dev: DRM device to be added
> >> - * @head: DRM head in question
> >> + * drm_sysfs_minor_alloc() - Allocate sysfs device for given minor
> >> + * @minor: minor to allocate sysfs device for
> >>   *
> >> - * Add a DRM device to the DRM's device model class.  We use @dev's PCI device
> >> - * as the parent for the Linux device, and make sure it has a file containing
> >> - * the driver we're using (for userspace compatibility).
> >> + * This allocates a new sysfs device for @minor and returns it. The device is
> >> + * not registered nor linked. The caller has to use device_add() and
> >> + * device_del() to register and unregister it.
> >> + *
> >> + * Note that dev_get_drvdata() on the new device will return the minor.
> >> + * However, the device does not hold a ref-count to the minor nor to the
> >> + * underlying drm_device. This is unproblematic as long as you access the
> >> + * private data only in sysfs callbacks. device_del() disables those
> >> + * synchronously, so they cannot be called after you cleanup a minor.
> >>   */
> >> -int drm_sysfs_device_add(struct drm_minor *minor)
> >> +struct device *drm_sysfs_minor_alloc(struct drm_minor *minor)
> >>  {
> >> -     char *minor_str;
> >> +     const char *minor_str;
> >> +     struct device *kdev;
> >>       int r;
> >>
> >>       if (minor->type == DRM_MINOR_CONTROL)
> >>               minor_str = "controlD%d";
> >> -        else if (minor->type == DRM_MINOR_RENDER)
> >> -                minor_str = "renderD%d";
> >> -        else
> >> -                minor_str = "card%d";
> >> -
> >> -     minor->kdev = kzalloc(sizeof(*minor->kdev), GFP_KERNEL);
> >> -     if (!minor->kdev) {
> >> -             r = -ENOMEM;
> >> -             goto error;
> >> -     }
> >> -
> >> -     device_initialize(minor->kdev);
> >> -     minor->kdev->devt = MKDEV(DRM_MAJOR, minor->index);
> >> -     minor->kdev->class = drm_class;
> >> -     minor->kdev->type = &drm_sysfs_device_minor;
> >> -     minor->kdev->parent = minor->dev->dev;
> >> -     minor->kdev->release = drm_sysfs_release;
> >> -     dev_set_drvdata(minor->kdev, minor);
> >> -
> >> -     r = dev_set_name(minor->kdev, minor_str, minor->index);
> >> +     else if (minor->type == DRM_MINOR_RENDER)
> >> +             minor_str = "renderD%d";
> >> +     else
> >> +             minor_str = "card%d";
> >> +
> >> +     kdev = kzalloc(sizeof(*kdev), GFP_KERNEL);
> >> +     if (!kdev)
> >> +             return ERR_PTR(-ENOMEM);
> >
> > Not sure about the device model, but could we just embedded the kdev into
> > struct drm_minor? Separate patch ofc.
> 
> I wouldn't do that. If we embed "struct device", then we _have_ to use
> the ref-counting of it (get_device(), put_device()) as all generic
> devices are ref-counted. I'd really like to avoid that. We make sure
> no-one uses dev_get_drvdata() on a device except for sysfs attribute
> handlers. Those handlers are removed synchronously during
> device_del(). Therefore, we can be sure even if the device lives
> longer than it's parent "struct drm_minor", no-one accesses the
> private data. We can thus release the kdev and free the minor early.
> The kdev will hang around until all refs are dropped, but it's
> detached from our drm_minor (that's why I do the drm_set_drvdata(kdev,
> NULL) after device_del() as safety-belt).

Hm yeah, makes sense. r-b still applies ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch