linux-kernel - Re: [PATCH 02/24] thunderbolt: Do not try to read UID if DROM offset is read as 0

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Date:   Sun, 21 May 2017 15:46:20 +0200
From:   Andreas Noever <andreas.noever@...il.com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <yehezkel.bernat@...el.com>,
        Lukas Wunner <lukas@...ner.de>,
        Amir Levy <amir.jer.levy@...el.com>,
        Andy Lutomirski <luto@...nel.org>, Mario.Limonciello@...l.com,
        Jared.Dominguez@...l.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/24] thunderbolt: Do not try to read UID if DROM offset
 is read as 0

On Thu, May 18, 2017 at 4:38 PM, Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
> At least Falcon Ridge when in host mode does not have any kind of DROM
> available and reading DROM offset returns 0 for these. Do not try to
> read DROM any further in that case.
>
> Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Reviewed-by: Yehezkel Bernat <yehezkel.bernat@...el.com>
> Reviewed-by: Michael Jamet <michael.jamet@...el.com>
> ---
>  drivers/thunderbolt/eeprom.c | 3 +++
>  1 file changed, 3 insertions(+)
>

Hi Mika,

nice work, it is nice to see Intel contribute to the Thunderbolt
driver (I can second Lukas's 'jaw drop' comment)!

I will try to read through everything today, but maybe the last few
patches will get pushed back to next weekend.

> diff --git a/drivers/thunderbolt/eeprom.c b/drivers/thunderbolt/eeprom.c
> index 6392990c984d..e4e64b130514 100644
> --- a/drivers/thunderbolt/eeprom.c
> +++ b/drivers/thunderbolt/eeprom.c
> @@ -276,6 +276,9 @@ int tb_drom_read_uid_only(struct tb_switch *sw, u64 *uid)
>         if (res)
>                 return res;
>
> +       if (drom_offset == 0)
> +               return -ENODEV;
> +
I think that this will make tb_switch_resume bail out on the root
switch, which is not good. Since the uid is only used to detect
whether a different device was plugged in while the system was
suspended I think that we can safely ignore the uid on the root
switch:
 - don't read it in tb_drom_read (route == 0 is already special cased anyways)
 - add a special case for the root switch to tb_switch_resume and
don't read the uid - just assume that it did not change (should be
impossible anyways)

What do you think?

Andreas

>         /* read uid */
>         res = tb_eeprom_read_n(sw, drom_offset, data, 9);
>         if (res)
> --
> 2.11.0
>