RE: [PATCH 3/3] drm/dp_mst: Remove single tx msg restriction.

["Lin, Wayne" <Wayne.Lin@amd.com>]

[AMD Public Use]



> -----Original Message-----
> From: Sean Paul <sean@poorly.run>
> Sent: Wednesday, February 19, 2020 1:15 AM
> To: Lin, Wayne <Wayne.Lin@amd.com>
> Cc: Sean Paul <sean@poorly.run>; dri-devel@lists.freedesktop.org;
> lyude@redhat.com; Sean Paul <seanpaul@chromium.org>; Maarten Lankhorst
> <maarten.lankhorst@linux.intel.com>; Maxime Ripard <mripard@kernel.org>;
> David Airlie <airlied@linux.ie>
> Subject: Re: [PATCH 3/3] drm/dp_mst: Remove single tx msg restriction.
> 
> On Tue, Feb 18, 2020 at 10:52:06AM -0500, Sean Paul wrote:
> > On Mon, Feb 17, 2020 at 07:08:37AM +0000, Lin, Wayne wrote:
> > > [AMD Public Use]
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Sean Paul <sean@poorly.run>
> > > > Sent: Saturday, February 15, 2020 12:09 AM
> > > > To: Lin, Wayne <Wayne.Lin@amd.com>
> > > > Cc: dri-devel@lists.freedesktop.org; lyude@redhat.com; Sean Paul
> > > > <seanpaul@chromium.org>; Maarten Lankhorst
> > > > <maarten.lankhorst@linux.intel.com>; Maxime Ripard
> > > > <mripard@kernel.org>; David Airlie <airlied@linux.ie>
> > > > Subject: Re: [PATCH 3/3] drm/dp_mst: Remove single tx msg restriction.
> > > >
> > > > On Fri, Feb 14, 2020 at 12:58 AM Lin, Wayne <Wayne.Lin@amd.com>
> wrote:
> > > > >
> > > > > [AMD Public Use]
> > > > >
> > > > > Hi Paul,
> > > > >
> > > > > Thanks for the mail!
> > > > >
> > > > > I tried to solve this problem by having restriction on sending
> > > > > one msg at a
> > > > time due to hub/dock compatibility problems.
> > > > > From my experience, some branch devices don't handle well on
> > > > > interleaved replies (Dock from HP I think)
> > > >
> > > > Hi Wayne,
> > > > Hmm, that's interesting, do you have a part number of the failing
> > > > dock so I can test it?
> > > >
> > > Hi Paul,
> > >
> > > Sorry but it's been quite a while. I can't exactly tell the part number.
> > > If I remember correctly, when the specific branch device receives
> > > interleaved replies, it just doesn't reply to any requests.
> > >
> > > > > As the result of that, correct me if I'm wrong, I remember most
> > > > > gpu vendors
> > > > just send one down request at a time now in windows environment.
> > > > > I would suggest the original solution :)
> > > >
> > > > I can't really say what happens on the Windows side of the world,
> > > > but I suppose that makes sense if this is a widespread issue with
> > > > docks. I do worry about the performance hit.
> > > >
> > > > If indeed this is a problem, could we ratelimit per branch device
> > > > instead of globally? Even that would be better than serializing everything.
> > > >
> > > Since the problem was because some branch devices can't
> > > simultaneously handle two replies, I'm afraid that we might still encounter
> the same problem?
> > >
> >
> > Hi Wayne,
> > Thanks for clarifying. I'm a bit hesitant to scrap this idea without
> > solid evidence that this is a common problem. Do you have any hubs
> > around AMD that you could try my patchset with?
Hi Paul,
Sure! I will see what I have at hand and try your patch set. It might take
me some time but I will update this as soon as possible. :)

Thanks!
> 
> Oh, if you can test the set, you'll also need this patch as well :-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> b/drivers/gpu/drm/drm_dp_mst_topology.c
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -3814,7 +3814,9 @@ static bool drm_dp_get_one_sb_msg(struct
> drm_dp_mst_topology_mgr *mgr, bool up,
>         int basereg = up ? DP_SIDEBAND_MSG_UP_REQ_BASE :
>                            DP_SIDEBAND_MSG_DOWN_REP_BASE;
> 
> -       *mstb = NULL;
> +       if (mstb)
> +               *mstb = NULL;
> +
>         *seqno = -1;
> 
>         len = min(mgr->max_dpcd_transaction_bytes, 16);
> 
> 
> > Perhaps we could get some hard data? I'm happy to test things on my
> > end, but I probably shouldn't just start buying a bunch of random HP
> > docks in hopes one of them exhibits the issue :)
> >
> > To add anecdote to anecdote, everything on my desk seems to work with
> > interleaved replies.
> >
> > Since HDCP spec requires the host to verify each channel's encryption
> > every <2s, we're going to be increasing the amount of sideband traffic
> > a fair amount, so I would like to ensure we're doing everything
> > possible to maximize our sideband bandwidth.
> >
> > Sean
> >
> > > Thanks!
> > > > Sean
> > > >
> > > > >
> > > > > Thanks!
> > > > > > -----Original Message-----
> > > > > > From: Sean Paul <sean@poorly.run>
> > > > > > Sent: Friday, February 14, 2020 5:15 AM
> > > > > > To: dri-devel@lists.freedesktop.org
> > > > > > Cc: lyude@redhat.com; Lin, Wayne <Wayne.Lin@amd.com>; Sean
> > > > > > Paul <seanpaul@chromium.org>; Maarten Lankhorst
> > > > > > <maarten.lankhorst@linux.intel.com>; Maxime Ripard
> > > > > > <mripard@kernel.org>; David Airlie <airlied@linux.ie>
> > > > > > Subject: [PATCH 3/3] drm/dp_mst: Remove single tx msg restriction.
> > > > > >
> > > > > > From: Sean Paul <seanpaul@chromium.org>
> > > > > >
> > > > > > Now that we can support multiple simultaneous replies, remove
> > > > > > the restrictions placed on sending new tx msgs.
> > > > > >
> > > > > > This patch essentially just reverts commit
> > > > > >   5a64967a2f3b ("drm/dp_mst: Have DP_Tx send one msg at a
> > > > > > time")
> > > > now
> > > > > > that the problem is solved in a different way.
> > > > > >
> > > > > > Cc: Wayne Lin <Wayne.Lin@amd.com>
> > > > > > Signed-off-by: Sean Paul <seanpaul@chromium.org>
> > > > > > ---
> > > > > >  drivers/gpu/drm/drm_dp_mst_topology.c | 14 ++------------
> > > > > >  include/drm/drm_dp_mst_helper.h       |  6 ------
> > > > > >  2 files changed, 2 insertions(+), 18 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > index 7e6a82efdfc02..cbf0bb0ddeb84 100644
> > > > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > > > @@ -1203,8 +1203,6 @@ static int
> > > > > > drm_dp_mst_wait_tx_reply(struct drm_dp_mst_branch *mstb,
> > > > > >                   txmsg->state ==
> DRM_DP_SIDEBAND_TX_SENT) {
> > > > > >                       mstb->tx_slots[txmsg->seqno] = NULL;
> > > > > >               }
> > > > > > -             mgr->is_waiting_for_dwn_reply = false;
> > > > > > -
> > > > > >       }
> > > > > >  out:
> > > > > >       if (unlikely(ret == -EIO) &&
> > > > > > drm_debug_enabled(DRM_UT_DP)) { @@
> > > > > > -1214,7 +1212,6 @@ static int drm_dp_mst_wait_tx_reply(struct
> > > > > > drm_dp_mst_branch *mstb,
> > > > > >       }
> > > > > >       mutex_unlock(&mgr->qlock);
> > > > > >
> > > > > > -     drm_dp_mst_kick_tx(mgr);
> > > > > >       return ret;
> > > > > >  }
> > > > > >
> > > > > > @@ -2797,11 +2794,9 @@ static void
> > > > > > process_single_down_tx_qlock(struct
> > > > > > drm_dp_mst_topology_mgr *mgr)
> > > > > >       ret = process_single_tx_qlock(mgr, txmsg, false);
> > > > > >       if (ret == 1) {
> > > > > >               /* txmsg is sent it should be in the slots now */
> > > > > > -             mgr->is_waiting_for_dwn_reply = true;
> > > > > >               list_del(&txmsg->next);
> > > > > >       } else if (ret) {
> > > > > >               DRM_DEBUG_KMS("failed to send msg in q %d\n",
> ret);
> > > > > > -             mgr->is_waiting_for_dwn_reply = false;
> > > > > >               list_del(&txmsg->next);
> > > > > >               if (txmsg->seqno != -1)
> > > > > >                       txmsg->dst->tx_slots[txmsg->seqno] =
> > > > > > NULL;
> > > > @@
> > > > > > -2841,8
> > > > > > +2836,7 @@ static void drm_dp_queue_down_tx(struct
> > > > > > drm_dp_mst_topology_mgr *mgr,
> > > > > >               drm_dp_mst_dump_sideband_msg_tx(&p, txmsg);
> > > > > >       }
> > > > > >
> > > > > > -     if (list_is_singular(&mgr->tx_msg_downq) &&
> > > > > > -         !mgr->is_waiting_for_dwn_reply)
> > > > > > +     if (list_is_singular(&mgr->tx_msg_downq))
> > > > > >               process_single_down_tx_qlock(mgr);
> > > > > >       mutex_unlock(&mgr->qlock);  } @@ -3822,7 +3816,6 @@
> > > > > > static int drm_dp_mst_handle_down_rep(struct
> > > > > > drm_dp_mst_topology_mgr *mgr)
> > > > > >       mutex_lock(&mgr->qlock);
> > > > > >       txmsg->state = DRM_DP_SIDEBAND_TX_RX;
> > > > > >       mstb->tx_slots[seqno] = NULL;
> > > > > > -     mgr->is_waiting_for_dwn_reply = false;
> > > > > >       mutex_unlock(&mgr->qlock);
> > > > > >
> > > > > >       wake_up_all(&mgr->tx_waitq); @@ -3830,9 +3823,6 @@
> > > > > > static int drm_dp_mst_handle_down_rep(struct
> > > > > > drm_dp_mst_topology_mgr *mgr)
> > > > > >       return 0;
> > > > > >
> > > > > >  out_clear_reply:
> > > > > > -     mutex_lock(&mgr->qlock);
> > > > > > -     mgr->is_waiting_for_dwn_reply = false;
> > > > > > -     mutex_unlock(&mgr->qlock);
> > > > > >       if (msg)
> > > > > >               memset(msg, 0, sizeof(struct
> > > > drm_dp_sideband_msg_rx));
> > > > > >  out:
> > > > > > @@ -4670,7 +4660,7 @@ static void drm_dp_tx_work(struct
> > > > > > work_struct
> > > > > > *work)
> > > > > >       struct drm_dp_mst_topology_mgr *mgr =
> container_of(work,
> > > > > > struct drm_dp_mst_topology_mgr, tx_work);
> > > > > >
> > > > > >       mutex_lock(&mgr->qlock);
> > > > > > -     if (!list_empty(&mgr->tx_msg_downq)
> > > > > > && !mgr->is_waiting_for_dwn_reply)
> > > > > > +     if (!list_empty(&mgr->tx_msg_downq))
> > > > > >               process_single_down_tx_qlock(mgr);
> > > > > >       mutex_unlock(&mgr->qlock);  } diff --git
> > > > > > a/include/drm/drm_dp_mst_helper.h
> > > > > > b/include/drm/drm_dp_mst_helper.h index
> > > > 7d0341f94ce1b..fcc30e64c8e7e
> > > > > > 100644
> > > > > > --- a/include/drm/drm_dp_mst_helper.h
> > > > > > +++ b/include/drm/drm_dp_mst_helper.h
> > > > > > @@ -619,12 +619,6 @@ struct drm_dp_mst_topology_mgr {
> > > > > >        * &drm_dp_sideband_msg_tx.state once they are queued
> > > > > >        */
> > > > > >       struct mutex qlock;
> > > > > > -
> > > > > > -     /**
> > > > > > -      * @is_waiting_for_dwn_reply: indicate whether is waiting
> for
> > > > down
> > > > > > reply
> > > > > > -      */
> > > > > > -     bool is_waiting_for_dwn_reply;
> > > > > > -
> > > > > >       /**
> > > > > >        * @tx_msg_downq: List of pending down replies.
> > > > > >        */
> > > > > > --
> > > > > > Sean Paul, Software Engineer, Google / Chromium OS
> > > > > --
> > > > > Wayne Lin
> > > --
> > > Best regards,
> > > Wayne Lin
> >
> > --
> > Sean Paul, Software Engineer, Google / Chromium OS
> 
> --
> Sean Paul, Software Engineer, Google / Chromium OS
--
Wayne Lin
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