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

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Date:   Thu, 1 Feb 2018 16:43:51 -0800
From:   Shannon Nelson <shannon.nelson@...cle.com>
To:     "Tantilov, Emil S" <emil.s.tantilov@...el.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

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