[PATCH 3/3] drm/mst: adjust the function drm_dp_remove_payload_part2()

[Lin, Wayne]

[AMD Official Use Only - General]

> -----Original Message-----
> From: Imre Deak <imre.deak at intel.com>
> Sent: Tuesday, August 8, 2023 12:00 AM
> To: Lin, Wayne <Wayne.Lin at amd.com>
> Cc: dri-devel at lists.freedesktop.org; amd-gfx at lists.freedesktop.org;
> lyude at redhat.com; jani.nikula at intel.com; ville.syrjala at linux.intel.com;
> Wentland, Harry <Harry.Wentland at amd.com>; Zuo, Jerry
> <Jerry.Zuo at amd.com>
> Subject: Re: [PATCH 3/3] drm/mst: adjust the function
> drm_dp_remove_payload_part2()
>
> On Mon, Aug 07, 2023 at 02:43:02AM +0000, Lin, Wayne wrote:
> > [AMD Official Use Only - General]
> >
> > > -----Original Message-----
> > > From: Imre Deak <imre.deak at intel.com>
> > > Sent: Friday, August 4, 2023 11:32 PM
> > > To: Lin, Wayne <Wayne.Lin at amd.com>
> > > Cc: dri-devel at lists.freedesktop.org; amd-gfx at lists.freedesktop.org;
> > > lyude at redhat.com; jani.nikula at intel.com;
> > > ville.syrjala at linux.intel.com; Wentland, Harry
> > > <Harry.Wentland at amd.com>; Zuo, Jerry <Jerry.Zuo at amd.com>
> > > Subject: Re: [PATCH 3/3] drm/mst: adjust the function
> > > drm_dp_remove_payload_part2()
> > >
> > > On Fri, Aug 04, 2023 at 02:20:29PM +0800, Wayne Lin wrote:
> > > > [...]
> > > > diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > > > b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > > > index e04f87ff755a..4270178f95f6 100644
> > > > --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > > > +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > > > @@ -3382,8 +3382,7 @@
> > > EXPORT_SYMBOL(drm_dp_remove_payload_part1);
> > > >   * drm_dp_remove_payload_part2() - Remove an MST payload locally
> > > >   * @mgr: Manager to use.
> > > >   * @mst_state: The MST atomic state
> > > > - * @old_payload: The payload with its old state
> > > > - * @new_payload: The payload with its latest state
> > > > + * @payload: The payload with its latest state
> > > >   *
> > > >   * Updates the starting time slots of all other payloads which
> > > > would have
> > > been shifted towards
> > > >   * the start of the payload ID table as a result of removing a
> > > > payload. Driver should call this @@ -3392,25 +3391,36 @@
> > > EXPORT_SYMBOL(drm_dp_remove_payload_part1);
> > > >   */
> > > >  void drm_dp_remove_payload_part2(struct
> drm_dp_mst_topology_mgr
> > > *mgr,
> > > >                              struct drm_dp_mst_topology_state
> > > *mst_state,
> > > > -                            const struct drm_dp_mst_atomic_payload
> > > *old_payload,
> > > > -                            struct drm_dp_mst_atomic_payload
> > > *new_payload)
> > > > +                            struct drm_dp_mst_atomic_payload
> > > *payload)
> > > >  {
> > > >     struct drm_dp_mst_atomic_payload *pos;
> > > > +   u8 time_slots_to_remove;
> > > > +   u8 next_payload_vc_start = mgr->next_start_slot;
> > > > +
> > > > +   /* Find the current allocated time slot number of the payload */
> > > > +   list_for_each_entry(pos, &mst_state->payloads, next) {
> > > > +           if (pos != payload &&
> > > > +               pos->vc_start_slot > payload->vc_start_slot &&
> > > > +               pos->vc_start_slot < next_payload_vc_start)
> > > > +                   next_payload_vc_start = pos->vc_start_slot;
> > > > +   }
> > > > +
> > > > +   time_slots_to_remove = next_payload_vc_start -
> > > > +payload->vc_start_slot;
> > >
> > > Imo, the intuitive way would be to pass the old payload state to
> > > this function - which already contains the required time_slots param
> > > - and refactor things instead moving vc_start_slot from the payload
> > > state to mgr suggested by Ville earlier.
> > >
> > > --Imre
> >
> > Hi Imre,
> > Thanks for your feedback!
> >
> > I understand it's functionally correct. But IMHO, it's still a bit
> > conceptually different between the time slot in old state and the time
> > slot in current payload table. My thought is the time slot at the
> > moment when we are removing the payload would be a better choice.
>
> Yes, they are different. The old state contains the time slot the payload was
> added with in a preceding commit and so the time slot value which should be
> used when removing the same payload in the current commit.
>
> The new state contains a time slot value with which the payload will be added
> in the current commit and can be different than the one in the old state if the
> current commit has changed the payload size (meaning that the same atomic
> commit will first remove the payload using the time slot value in the old state
> and afterwards will add back the same payload using the time slot value in the
> new state).
>
Appreciate your time, Imre!

Yes I understand, so I'm not using the number of the time slot in the new state.
I'm referring to the start slot instead which is updated during every allocation
and removement at current commit.

Like what you said, current commit manipulation could be a mix of allocations
and removements for the payload. My thought is, conceptually, looking up the
latest number of time slot is a better choice rather than the one in old state.
It's relatively intuitive to me since we are removing payload from current
payload table and which changes since last preceding commit till the moment
we're deleting the payload. Although it's unreasonable that these values are
different.

> > And with this, we can also simplify some codes. Especially remove
> > workaround in amd driver. In fact, DRM mst code maintains the payload
> > table and all the time slot info is in it already. We don't really
> > have to pass a new parameter.
>
> I agree that drm_dp_remove_payload() could be simplified, but this should be
> done so that the drivers can pass the old payload state to it (without having to
> pass the new state). This would be possible if vc_start_slot was not tracked in
> the payload state (which is really not an atomic state that can be precomputed
> as all other atomic state), rather it would be tracked in struct
> drm_dp_mst_topology_mgr.
>

So the reason I chose to pass the new state is like what I mentioned above. I
would prefer to carry the latest updated payload table instead which is in the new
state. And I agree with the explanation for the vc_start_slot and that's also my
thought at the beginning. It could be a refactor later, but no matter the start slot
is put into payload state or the topology manager I would prefer to refer to the
latest payload table rather than the number of time slot in the old state.

> It looks like AMD has to reconstruct the old state in
> dm_helpers_construct_old_payload(). Could you explain why it couldn't
> instead just pass old_payload acquired by
>
> old_mst_state = drm_atomic_get_old_mst_topology_state();
> old_payload = drm_atomic_get_mst_payload_state(old_mst_state);
>
> ?

AMD doesn't pass the drm old state to the stage while HW is deleting the payload.
The reason is that HW payload table is known during HW programming procedure,
so the payload removement is based on the table at the moment. AMD expected
the current number of time slot is also already maintained in drm layer.

Again, thanks for your feedback Imre!

>
> > > >     /* Remove local payload allocation */
> > > >     list_for_each_entry(pos, &mst_state->payloads, next) {
> > > > -           if (pos != new_payload && pos->vc_start_slot > new_payload-
> > > >vc_start_slot)
> > > > -                   pos->vc_start_slot -= old_payload->time_slots;
> > > > +           if (pos != payload && pos->vc_start_slot > payload-
> > > >vc_start_slot)
> > > > +                   pos->vc_start_slot -= time_slots_to_remove;
> > > >     }
> > > > -   new_payload->vc_start_slot = -1;
> > > > +   payload->vc_start_slot = -1;
> > > >
> > > >     mgr->payload_count--;
> > > > -   mgr->next_start_slot -= old_payload->time_slots;
> > > > +   mgr->next_start_slot -= time_slots_to_remove;
> > > >
> > > > -   if (new_payload->delete)
> > > > -           drm_dp_mst_put_port_malloc(new_payload->port);
> > > > +   if (payload->delete)
> > > > +           drm_dp_mst_put_port_malloc(payload->port);
> > > >
> > > > -   new_payload->payload_allocation_status =
> > > DRM_DP_MST_PAYLOAD_ALLOCATION_NONE;
> > > > +   payload->payload_allocation_status =
> > > > +DRM_DP_MST_PAYLOAD_ALLOCATION_NONE;
> > > >  }
> >
> > --
> > Regards,
> > Wayne

--
Regards,
Wayne