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SubjectRe: [PATCH v2 16/28] thunderbolt: Discover preboot PCIe paths the boot firmware established
On Wed, Feb 06, 2019 at 04:17:26PM +0300, Mika Westerberg wrote:
> /* dword 0 */
> hop.next_hop = path->hops[i].next_hop_index;
> hop.out_port = path->hops[i].out_port->port;
> - /* TODO: figure out why these are good values */
> - hop.initial_credits = (i == path->path_length - 1) ? 16 : 7;
> + hop.initial_credits = path->hops[i].initial_credits;
> hop.unknown1 = 0;
> hop.enable = 1;
[...]
> @@ -78,6 +78,74 @@ static void tb_pci_init_path(struct tb_path *path)
> path->weight = 1;
> path->drop_packages = 0;
> path->nfc_credits = 0;
> + path->hops[0].initial_credits = 7;
> + path->hops[1].initial_credits = 16;

I guess Andreas' algorithm (the last hop in the path is assigned
16 and all the ones before are assigned 7) was reverse-engineered
from Apple's driver. The fact that this algorithm works for paths
of arbitrary length could indicate that Apple indeed does establish
tunnels between non-adjacent switches.

Also, why are these good values? (You've deleted the comment.)

Thanks,

Lukas

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