Re: [PATCH 2/3] drm: clarify lifetime/locking for drm_master's lease fields

[Daniel Vetter]

On Thu, Jul 22, 2021 at 3:03 PM Desmond Cheong Zhi Xi
<desmondcheongzx@xxxxxxxxx> wrote:
>
> On 22/7/21 6:35 pm, Daniel Vetter wrote:
> > On Thu, Jul 22, 2021 at 05:29:28PM +0800, Desmond Cheong Zhi Xi wrote:
> >> In particular, we make it clear that &drm_device.mode_config.idr_mutex
> >> protects the lease idr and list structures for drm_master. The lessor
> >> field itself doesn't need to be protected as it doesn't change after
> >> it's set in drm_lease_create.
> >>
> >> Additionally, we add descriptions for the lifetime of lessors and
> >> leases to make it easier to reason about them.
> >>
> >> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> >> ---
> >>   include/drm/drm_auth.h | 62 ++++++++++++++++++++++++++++++++++--------
> >>   1 file changed, 51 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h
> >> index f99d3417f304..c978c85464fa 100644
> >> --- a/include/drm/drm_auth.h
> >> +++ b/include/drm/drm_auth.h
> >> @@ -58,12 +58,6 @@ struct drm_lock_data {
> >>    * @refcount: Refcount for this master object.
> >>    * @dev: Link back to the DRM device
> >>    * @driver_priv: Pointer to driver-private information.
> >> - * @lessor: Lease holder
> >> - * @lessee_id: id for lessees. Owners always have id 0
> >> - * @lessee_list: other lessees of the same master
> >> - * @lessees: drm_masters leasing from this one
> >> - * @leases: Objects leased to this drm_master.
> >> - * @lessee_idr: All lessees under this owner (only used where lessor == NULL)
> >>    *
> >>    * Note that master structures are only relevant for the legacy/primary device
> >>    * nodes, hence there can only be one per device, not one per drm_minor.
> >> @@ -88,17 +82,63 @@ struct drm_master {
> >>      struct idr magic_map;
> >>      void *driver_priv;
> >>
> >> -    /* Tree of display resource leases, each of which is a drm_master struct
> >> -     * All of these get activated simultaneously, so drm_device master points
> >> -     * at the top of the tree (for which lessor is NULL). Protected by
> >> -     * &drm_device.mode_config.idr_mutex.
> >> +    /**
> >> +     * @lessor:
> >> +     *
> >> +     * Lease holder. The lessor does not change once it's set in
> >
> > Lessor is the lease grantor, lessee is the one receiving the lease. Maybe
> > clarify this with
> >
> > "Lease grantor, only set if this struct drm_master represents a lessee
> > holding a lease of objects from @lessor. Full owners of the device have
> > this set to NULL."
> >
> >> +     * drm_lease_create(). Each lessee holds a reference to its lessor that
> >
> > I also figured it'd be a good idea to link to the drm_lease docs here to
> > explain the concepts, but alas we don't have those :-( Hence at least
> > define what we mean with lessor, lessee, lease and owner.
> >
>
> Thanks for the suggestions, Daniel. I'll incorporate them in a v2.
>
> Regarding the missing drm_lease docs... any reason we can't just add it
> in? Seems like most of the comments in drm_lease.c are almost correctly
> formatted too. I could clean them up, define the terms in a preamble,
> then add it to the v2 patch.

Sure if you want to do that, that would be great. Usual style tips:
- We only document driver interfaces, so structs/inline functions in
headers and exported symbols in .c files.
- Anything else interesting just leave as normal C comments
- An overview DOC: section that explains a bit how leases work and why
(git blame on the commit that added it should help, otherwise I can
type that up) would also be really great.

I think the right place for this is in the drm-uapi.rst section after
the section about primary nodes:

https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html#modeset-base-object-abstraction

Cheers, Daniel


>
> >> +     * it releases upon being destroyed in drm_lease_destroy().
> >> +     *
> >> +     * Display resource leases form a tree of &struct drm_master. All of
> >
> > For now we've limited the tree to a depth of 1, you can't create another
> > lease if yourself are a lease. I guess another reason to update the
> > drm_lease.c docs.
> >
> > So maybe add "Currently the lease tree depth is limited to 1."
> >
> >> +     * these get activated simultaneously, so &drm_device.master
> >> +     * points at the top of the tree (for which lessor is NULL).
> >>       */
> >> -
> >>      struct drm_master *lessor;
> >> +
> >> +    /**
> >> +     * @lessee_id:
> >> +     *
> >> +     * ID for lessees. Owners always have ID 0. Protected by
> >
> > Maybe clarify to "Owners (i.e. @lessor is NULL) ..."
> >
> >> +     * &drm_device.mode_config's &drm_mode_config.idr_mutex.
> >> +     */
> >>      int     lessee_id;
> >> +
> >> +    /**
> >> +     * @lessee_list:
> >> +     *
> >> +     * List of lessees of the same master. Protected by
> >
> > We try to distinguis between list entries and the list, even though it's
> > the same struct. So maybe
> >
> > "List entry of lessees of @lessor, where they are linked to @lessee. Not
> > use for owners."
> >
> >> +     * &drm_device.mode_config's &drm_mode_config.idr_mutex.
> >
> >> +     */
> >>      struct list_head lessee_list;
> >> +
> >> +    /**
> >> +     * @lessees:
> >> +     *
> >> +     * List of drm_masters leasing from this one. Protected by
> >> +     * &drm_device.mode_config's &drm_mode_config.idr_mutex.
> >> +     *
> >> +     * This master cannot be destroyed unless this list is empty as lessors
> >> +     * are referenced by all their lessees.
> >
> > Maybe add "this list is empty of no leases have been granted."
> >
> >> +     */
> >>      struct list_head lessees;
> >> +
> >> +    /**
> >> +     * @leases:
> >> +     *
> >> +     * Objects leased to this drm_master. Protected by
> >> +     * &drm_device.mode_config's &drm_mode_config.idr_mutex.
> >> +     *
> >> +     * Objects are leased all together in drm_lease_create(), and are
> >> +     * removed all together when the lease is revoked.
> >> +     */
> >>      struct idr leases;
> >> +
> >> +    /**
> >> +     * @lessee_idr:
> >> +     *
> >> +     * All lessees under this owner (only used where lessor is NULL).
> >
> > @lessor so it's highlighted correctly
> >
> >> +     * Protected by &drm_device.mode_config's &drm_mode_config.idr_mutex.
> >> +     */
> >>      struct idr lessee_idr;
> >>      /* private: */
> >
> > With the nits addressed:
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> >
> > Thanks for updating the docs!
> > -Daniel
> >
> >>   #if IS_ENABLED(CONFIG_DRM_LEGACY)
> >> --
> >> 2.25.1
> >>
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch