linux-kernel - Re: [PATCH v2 16/28] thunderbolt: Discover preboot PCIe paths the boot firmware established

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Date:   Tue, 12 Feb 2019 18:49:42 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Andreas Noever <andreas.noever@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [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