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From:   Rob Clark <robdclark@gmail.com>
Date:   Wed, 21 Feb 2018 10:36:06 -0500
Message-ID: <CAF6AEGs+_gjEzueON9Rz8ce9xzHKhkdR2yQGfSpC=6DQV19V2A@mail.gmail.com>
Subject: Re: [PATCH 1/4] drm/atomic: integrate modeset lock with private objects
To:     =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        David Airlie <airlied@linux.ie>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Feb 21, 2018 at 10:27 AM, Ville Syrj=C3=A4l=C3=A4
<ville.syrjala@linux.intel.com> wrote:
> On Wed, Feb 21, 2018 at 10:20:03AM -0500, Rob Clark wrote:
>> On Wed, Feb 21, 2018 at 10:07 AM, Ville Syrj=C3=A4l=C3=A4
>> <ville.syrjala@linux.intel.com> wrote:
>> > On Wed, Feb 21, 2018 at 09:54:49AM -0500, Rob Clark wrote:
>> >> On Wed, Feb 21, 2018 at 9:49 AM, Ville Syrj=C3=A4l=C3=A4
>> >> <ville.syrjala@linux.intel.com> wrote:
>> >> > On Wed, Feb 21, 2018 at 09:37:21AM -0500, Rob Clark wrote:
>> >> >> Follow the same pattern of locking as with other state objects.  T=
his
>> >> >> avoids boilerplate in the driver.
>> >> >
>> >> > I'm not sure we really want to do this. What if the driver wants a
>> >> > custom locking scheme for this state?
>> >>
>> >> That seems like something we want to discourage, ie. all the more
>> >> reason for this patch.
>> >>
>> >> There is no reason drivers could not split their global state into
>> >> multiple private objs's, each with their own lock, for more fine
>> >> grained locking.  That is basically the only valid reason I can think
>> >> of for "custom locking".
>> >
>> > In i915 we have at least one case that would want something close to a=
n
>> > rwlock. Any crtc lock is enough for read, need all of them for write.
>> > Though if we wanted to use private objs for that we might need to
>> > actually make the states refcounted as well, otherwise I can imagine
>> > we might land in some use-after-free issues once again.
>> >
>> > Maybe we could duplicate the state into per-crtc and global copies, bu=
t
>> > then we have to keep all of those in sync somehow which doesn't sound
>> > particularly pleasant.
>>
>> Or just keep your own driver lock for read, and use that plus the core
>> modeset lock for write?
>
> If we can't add the private obj to the state we can't really use it.
>

I'm not sure why that is strictly true (that you need to add it to the
state if for read-only), since you'd be guarding it with your own
driver read-lock you can just priv->foo_state->bar.

Since it is read-only access, there is no roll-back to worry about for
test-only or failed atomic_check()s..

BR,
-R


>>
>> BR,
>> -R
>>
>> >
>> >>
>> >> (And ofc drivers could add there own locks in addition to what is don=
e
>> >> by core, but I'd rather look at that on a case by case basis, rather
>> >> than it being part of the boilerplate in each driver.)
>> >>
>> >> BR,
>> >> -R
>> >>
>> >> >>
>> >> >> Signed-off-by: Rob Clark <robdclark@gmail.com>
>> >> >> ---
>> >> >>  drivers/gpu/drm/drm_atomic.c | 9 ++++++++-
>> >> >>  include/drm/drm_atomic.h     | 5 +++++
>> >> >>  2 files changed, 13 insertions(+), 1 deletion(-)
>> >> >>
>> >> >> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_at=
omic.c
>> >> >> index fc8c4da409ff..004e621ab307 100644
>> >> >> --- a/drivers/gpu/drm/drm_atomic.c
>> >> >> +++ b/drivers/gpu/drm/drm_atomic.c
>> >> >> @@ -1078,6 +1078,8 @@ drm_atomic_private_obj_init(struct drm_priva=
te_obj *obj,
>> >> >>  {
>> >> >>       memset(obj, 0, sizeof(*obj));
>> >> >>
>> >> >> +     drm_modeset_lock_init(&obj->lock);
>> >> >> +
>> >> >>       obj->state =3D state;
>> >> >>       obj->funcs =3D funcs;
>> >> >>  }
>> >> >> @@ -1093,6 +1095,7 @@ void
>> >> >>  drm_atomic_private_obj_fini(struct drm_private_obj *obj)
>> >> >>  {
>> >> >>       obj->funcs->atomic_destroy_state(obj, obj->state);
>> >> >> +     drm_modeset_lock_fini(&obj->lock);
>> >> >>  }
>> >> >>  EXPORT_SYMBOL(drm_atomic_private_obj_fini);
>> >> >>
>> >> >> @@ -1113,7 +1116,7 @@ struct drm_private_state *
>> >> >>  drm_atomic_get_private_obj_state(struct drm_atomic_state *state,
>> >> >>                                struct drm_private_obj *obj)
>> >> >>  {
>> >> >> -     int index, num_objs, i;
>> >> >> +     int index, num_objs, i, ret;
>> >> >>       size_t size;
>> >> >>       struct __drm_private_objs_state *arr;
>> >> >>       struct drm_private_state *obj_state;
>> >> >> @@ -1122,6 +1125,10 @@ drm_atomic_get_private_obj_state(struct drm=
_atomic_state *state,
>> >> >>               if (obj =3D=3D state->private_objs[i].ptr)
>> >> >>                       return state->private_objs[i].state;
>> >> >>
>> >> >> +     ret =3D drm_modeset_lock(&obj->lock, state->acquire_ctx);
>> >> >> +     if (ret)
>> >> >> +             return ERR_PTR(ret);
>> >> >> +
>> >> >>       num_objs =3D state->num_private_objs + 1;
>> >> >>       size =3D sizeof(*state->private_objs) * num_objs;
>> >> >>       arr =3D krealloc(state->private_objs, size, GFP_KERNEL);
>> >> >> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
>> >> >> index 09076a625637..9ae53b73c9d2 100644
>> >> >> --- a/include/drm/drm_atomic.h
>> >> >> +++ b/include/drm/drm_atomic.h
>> >> >> @@ -218,6 +218,11 @@ struct drm_private_state_funcs {
>> >> >>   * &drm_modeset_lock is required to duplicate and update this obj=
ect's state.
>> >> >>   */
>> >> >>  struct drm_private_obj {
>> >> >> +     /**
>> >> >> +      * @lock: Modeset lock to protect the state object.
>> >> >> +      */
>> >> >> +     struct drm_modeset_lock lock;
>> >> >> +
>> >> >>       /**
>> >> >>        * @state: Current atomic state for this driver private obje=
ct.
>> >> >>        */
>> >> >> --
>> >> >> 2.14.3
>> >> >>
>> >> >> _______________________________________________
>> >> >> dri-devel mailing list
>> >> >> dri-devel@lists.freedesktop.org
>> >> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>> >> >
>> >> > --
>> >> > Ville Syrj=C3=A4l=C3=A4
>> >> > Intel OTC
>> >
>> > --
>> > Ville Syrj=C3=A4l=C3=A4
>> > Intel OTC
>
> --
> Ville Syrj=C3=A4l=C3=A4
> Intel OTC