Re: [PATCH 2/5] drm/i915/dp_tunnel: Simplify detection of link BW change

[Imre Deak <imre.deak@intel.com>]

On Mon, Feb 23, 2026 at 06:02:26PM +0200, Murthy, Arun R wrote:
> 
> > -----Original Message-----
> > From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf Of Imre
> > Deak
> > Sent: Thursday, February 19, 2026 11:58 PM
> > To: intel-gfx@lists.freedesktop.org; intel-xe@lists.freedesktop.org
> > Subject: [PATCH 2/5] drm/i915/dp_tunnel: Simplify detection of link BW change
> > 
> > update_tunnel_state() checks whether a tunnel state change (e.g.
> > available tunnel bandwidth) affects the list of valid modes for the sink
> > connected through the tunnel. If so, its caller sends a hotplug event so
> > userspace can re-enumerate the modes.
> > 
> > A change in tunnel bandwidth does not affect the mode list if the bandwidth
> > was above the sink's DPRX bandwidth both before and after the update, since in
> > that case the effective bandwidth remains limited by the DPRX.
> > 
> > As get_current_link_bw() via intel_dp_max_link_data_rate() already returns
> > bandwidth values clamped to the DPRX limit, the condition for detecting a
> > mode list change can be simplified to:
> > 
> >     old_bw != new_bw
> > 
> > Remove the explicit checks for whether the bandwidth was below the
> > maximum DPRX bandwidth before and after the update, and rely on the
> > clamped bandwidth values instead.
> > 
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_tunnel.c | 18 +++++-------------
> >  1 file changed, 5 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_tunnel.c
> > b/drivers/gpu/drm/i915/display/intel_dp_tunnel.c
> > index eb1eed1c8c7bb..9f3750035f68e 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_tunnel.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_tunnel.c
> > @@ -54,30 +54,23 @@ static int kbytes_to_mbits(int kbytes)
> >  	return DIV_ROUND_UP(kbytes * 8, 1000);  }
> > 
> > -static int get_current_link_bw(struct intel_dp *intel_dp,
> > -			       bool *below_dprx_bw)
> > +static int get_current_link_bw(struct intel_dp *intel_dp)
> >  {
> >  	int rate = intel_dp_max_common_rate(intel_dp);
> >  	int lane_count = intel_dp_max_common_lane_count(intel_dp);
> > -	int bw;
> > 
> > -	bw = intel_dp_max_link_data_rate(intel_dp, rate, lane_count);
> > -	*below_dprx_bw = bw < drm_dp_max_dprx_data_rate(rate,
> > lane_count);
> > -
> > -	return bw;
> > +	return intel_dp_max_link_data_rate(intel_dp, rate, lane_count);
> >  }
>
> Function name says get the current data rate, but we are returning the
> max data rate here.

It is the current link bandwidth, i.e., the link bandwidth allowed by
the Thunderbolt Connection Manager at the moment. It is not a maximum
(which could refer either to the maximum DPRX bandwidth or the maximum
Thunderbolt bandwidth), but rather the amount allowed by all other
components on the Thunderbolt link sharing the same bandwidth.

> 
> Thanks and Regards,
> Arun R Murthy
> -------------------
> > 
> >  static int update_tunnel_state(struct intel_dp *intel_dp)  {
> >  	struct intel_display *display = to_intel_display(intel_dp);
> >  	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
> > -	bool old_bw_below_dprx;
> > -	bool new_bw_below_dprx;
> >  	int old_bw;
> >  	int new_bw;
> >  	int ret;
> > 
> > -	old_bw = get_current_link_bw(intel_dp, &old_bw_below_dprx);
> > +	old_bw = get_current_link_bw(intel_dp);
> > 
> >  	ret = drm_dp_tunnel_update_state(intel_dp->tunnel);
> >  	if (ret < 0) {
> > @@ -96,11 +89,10 @@ static int update_tunnel_state(struct intel_dp
> > *intel_dp)
> > 
> >  	intel_dp_update_sink_caps(intel_dp);
> > 
> > -	new_bw = get_current_link_bw(intel_dp, &new_bw_below_dprx);
> > +	new_bw = get_current_link_bw(intel_dp);
> > 
> >  	/* Suppress the notification if the mode list can't change due to bw. */
> > -	if (old_bw_below_dprx == new_bw_below_dprx &&
> > -	    !new_bw_below_dprx)
> > +	if (old_bw == new_bw)
> >  		return 0;
> > 
> >  	drm_dbg_kms(display->drm,
> > --
> > 2.49.1
>