netdev - RE: [Intel-wired-lan] Possible read-modify-write bug in ixgbe x550 phy setup

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Date:   Fri, 2 Feb 2018 00:58:01 +0000
From:   "Tantilov, Emil S" <emil.s.tantilov@...el.com>
To:     Shannon Nelson <shannon.nelson@...cle.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
Subject: RE: [Intel-wired-lan] Possible read-modify-write bug in ixgbe x550
 phy setup

>-----Original Message-----
>From: Shannon Nelson [mailto:shannon.nelson@...cle.com]
>Sent: Thursday, February 01, 2018 4:44 PM
>To: Tantilov, Emil S <emil.s.tantilov@...el.com>
>Cc: netdev@...r.kernel.org; intel-wired-lan@...ts.osuosl.org
>Subject: Re: [Intel-wired-lan] Possible read-modify-write bug in ixgbe
>x550 phy setup
>
>On 2/1/2018 4:34 PM, Tantilov, Emil S wrote:
>>> -----Original Message-----
>>> From: Intel-wired-lan [mailto:intel-wired-lan-bounces@...osl.org] On
>>> Behalf Of Shannon Nelson
>>> Sent: Thursday, February 01, 2018 3:46 PM
>>> To: Tantilov, Emil S <emil.s.tantilov@...el.com>
>>> Cc: netdev@...r.kernel.org; intel-wired-lan@...ts.osuosl.org
>>> Subject: [Intel-wired-lan] Possible read-modify-write bug in ixgbe
>x550
>>> phy setup
>>>
>>> Hi Emil,
>>>
>>> I was looking through a set of ixgbe patches and came across this
>commit
>>>
>>>      commit 410a494902777c11f95031d9ed757d7f8f09c5c6
>>>      ixgbe: add write flush when configuring CS4223/7
>>>
>>> and am wondering about the setting of reg_phy_ext in the middle of
>>> ixgbe_setup_mac_link_sfp_x550a().  It looks like it is read from the
>>> PHY, modified to remove the CX1 and SR mode bits, but then those bits
>>> are overwritten in the "if (setup_linear)" block immediately
>following,
>>> and that is what gets written back out.
>>
>> Hi Shannon,
>>
>> This is pretty standard clear before set, so you're right that it
>would
>> make more sense to have |= rather than =.
>>
>> Are you seeing an issue, or did you catch this via code inspection?
>
>Purely a code inspection, I saw this while looking at something else.
>
>sln

Thanks for letting us know. I will do some more testing with the actual HW
before changing the code.

Emil