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

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Date:   Fri, 2 Feb 2018 00:34:15 +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: 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?

Thanks,
Emil