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thunderbolt: share an Apple host link between DP tunnels that come up together #154

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thunderbolt: share an Apple host link between DP tunnels that come up together #154

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59 changes: 59 additions & 0 deletions drivers/thunderbolt/tunnel.c
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Original file line number Diff line number Diff line change
Expand Up @@ -914,6 +914,53 @@ static int tb_dp_reduce_bandwidth(int max_bw, u32 in_rate, u32 in_lanes,
return -ENOSR;
}

/*
* The DP IN adapters of an Apple silicon host never report the trained link
* rate in DP_COMMON_CAP, so tb_dp_consumed_bandwidth() keeps charging a
* tunnel the full rate it was offered. When two DP OUT adapters of one
* router come up together, the two tiles of a 5K display or two monitors on
* a dock, the first tunnel is offered all that fits (HBR3 x4, 25.92 Gb/s of
* a 40 Gb/s link) and the second is left with HBR x4, too little for a
* 2560x2880@60 tile. Offer each of them an even share instead: 2 x HBR2 x4
* fits the 40 Gb/s link, and a host or sink that needs no sharing is not
* limited. A dock with two DP OUT adapters and one monitor gets half the
* link for that monitor (HBR2 x4 on 40 Gb/s, enough for 4K60 at 10 bpc).
*/
static bool apple_dpin_share_bandwidth = true;
module_param(apple_dpin_share_bandwidth, bool, 0444);
MODULE_PARM_DESC(apple_dpin_share_bandwidth,
"Offer a DP tunnel from an Apple host DP IN adapter an even share of the available bandwidth when other DP OUT adapters of the same router have no tunnel yet (default: true)");

/*
* DP OUT adapters of @out's router that share the bandwidth of the path to
* it with this tunnel, @out included: those with HPD asserted and no tunnel
* yet. The second tile of a Studio Display XDR raises HPD only after the
* first tile's link is trained, and a DP OUT adapter without a sink (the
* display's third one) never does, so the first DP tunnel to a router with
* more than one DP OUT adapter reserves room for one more instead of
* counting them.
*/
static int tb_dp_pending_sinks(struct tb_port *out)
{
struct tb_port *port;
int outs = 0, tunneled = 0, sinks = 0;

tb_switch_for_each_port(out->sw, port) {
if (!tb_port_is_dpout(port))
continue;
outs++;
if (tb_port_is_enabled(port))
tunneled++;
else if (port == out || tb_dp_port_hpd_is_active(port) > 0)
sinks++;
}

if (!tunneled && outs > 1 && sinks < 2)
sinks = 2;

return sinks;
}

static int tb_dp_xchg_caps(struct tb_tunnel *tunnel)
{
u32 out_dp_cap, out_rate, out_lanes, in_dp_cap, in_rate, in_lanes, bw;
Expand Down Expand Up @@ -975,6 +1022,18 @@ static int tb_dp_xchg_caps(struct tb_tunnel *tunnel)
else
max_bw = tunnel->max_up;

if (apple_dpin_share_bandwidth && max_bw > 0 &&
tb_port_is_apple_host_dpin(in)) {
int sinks = tb_dp_pending_sinks(out);

if (sinks > 1) {
max_bw /= sinks;
tb_tunnel_info(tunnel,
"sharing the path with %d DP OUT adapters, offering %d Mb/s\n",
sinks, max_bw);
}
}

if (max_bw && bw > max_bw) {
u32 new_rate, new_lanes, new_bw;

Expand Down