linux-kernel - Re: [PATCH v2 12/28] thunderbolt: Add functions for allocating and releasing hop IDs

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Message-ID: <20190212124333.hgrnm6owtuqzp4iu@wunner.de>
Date:   Tue, 12 Feb 2019 13:43:33 +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 12/28] thunderbolt: Add functions for allocating and
 releasing hop IDs

On Mon, Feb 11, 2019 at 10:30:43AM +0200, Mika Westerberg wrote:
> On Sun, Feb 10, 2019 at 01:13:53PM +0100, Lukas Wunner wrote:
> > If there are two Macs at the ends of the daisy-chain with Thunderbolt
> > devices in-between, the other Mac may already have established tunnels
> > to some of the devices and therefore has occupied hop entries in the
> > devices' path config space.  How do you ensure that you don't allocate
> > the same entries and overwrite the other Mac's hop entries, thereby
> > breaking its tunnels?
> 
> If the other Mac has enumerated the device (set the upstream port,
> route, depth) then the other Mac cannot access the device. You get an
> error (we deal with that in the later patch in the series when we
> identify XDomain connections). The Hop ID allocation is only relevant in
> a single domain.  Crossing one needs to have protocol such as we have in
> case of ThunderboltIP to negotiate Hop IDs used in the link between two
> domains.

Understood now, thanks.  (Well, in part at least.)

It looks like there's a race condition currently in tb_switch_configure()
wherein two machines on the daisy chain may write the config simultaneously
and overwrite each other's changes.  Isn't there some kind of synchonization
mechanism available to prevent such an outcome?

Thanks,

Lukas