netdev - Re: [PATCH 4/4] thunderbolt: Get rid of E2E workaround

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Date:   Mon, 15 Jun 2020 18:15:47 +0300
From:   Yehezkel Bernat <yehezkelshb@...il.com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, Michael Jamet <michael.jamet@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Andreas Noever <andreas.noever@...il.com>,
        Lukas Wunner <lukas@...ner.de>, netdev@...r.kernel.org
Subject: Re: [PATCH 4/4] thunderbolt: Get rid of E2E workaround

On Mon, Jun 15, 2020 at 5:22 PM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> On Mon, Jun 15, 2020 at 05:18:38PM +0300, Yehezkel Bernat wrote:
> > On Mon, Jun 15, 2020 at 4:51 PM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Mon, Jun 15, 2020 at 04:45:22PM +0300, Yehezkel Bernat wrote:
> > > > On Mon, Jun 15, 2020 at 4:02 PM Mika Westerberg
> > > > <mika.westerberg@...ux.intel.com> wrote:
> > > > >
> > > > > diff --git a/include/linux/thunderbolt.h b/include/linux/thunderbolt.h
> > > > > index ff397c0d5c07..5db2b11ab085 100644
> > > > > --- a/include/linux/thunderbolt.h
> > > > > +++ b/include/linux/thunderbolt.h
> > > > > @@ -504,8 +504,6 @@ struct tb_ring {
> > > > >  #define RING_FLAG_NO_SUSPEND   BIT(0)
> > > > >  /* Configure the ring to be in frame mode */
> > > > >  #define RING_FLAG_FRAME                BIT(1)
> > > > > -/* Enable end-to-end flow control */
> > > > > -#define RING_FLAG_E2E          BIT(2)
> > > > >
> > > >
> > > > Isn't it better to keep it (or mark it as reserved) so it'll not cause
> > > > compatibility issues with older versions of the driver or with Windows?
> > >
> > >
> > > How can you have "older versions of the driver"?  All drivers are in the
> > > kernel tree at the same time, you can't ever mix-and-match drivers and
> > > kernels.
> > >
> > > And how does Windows come into play here?
> > >
> >
> > As much as I remember, this flag is sent as part of creating the
> > interdomain connection.
> > If we reuse this bit to something else, and the other host runs an
> > older kernel or
> > Windows, this seems to be an issue.
> > But maybe I don't remember it correctly.
>
> We never send this flag anywhere. At the moment we do not announce
> support the "full E2E" in the network driver. Basically this is dead
> code what we remove.

OK, maybe we never sent it, but Windows driver does send such a flag somewhere.
This is the only way both sides can know both of them support it and that they
should start using it. I'd prefer at least leaving a comment here that mentions
this, so if someone goes to add flags in the future, they will know to
take it into consideration.