linux-kernel - Re: [PATCH] net: phy: Fix phy_modify() semantic difference fallout

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Message-ID: <CAMuHMdXBasJu-u54LxT3x3fXcnW4Bm9N1nNyt-S=R5+=3oZaDA@mail.gmail.com>
Date:   Tue, 9 Jan 2018 19:25:40 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "David S . Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: phy: Fix phy_modify() semantic difference fallout

Hi Russell,

On Tue, Jan 9, 2018 at 3:22 PM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Tue, Jan 09, 2018 at 03:10:08PM +0100, Andrew Lunn wrote:
>> On Tue, Jan 09, 2018 at 12:11:21PM +0100, Geert Uytterhoeven wrote:
>> > In case of success, the return values of (__)phy_write() and
>> > (__)phy_modify() are not compatible: (__)phy_write() returns 0, while
>> > (__)phy_modify() returns the old PHY register value.
>> >
>> > Apparently this change was catered for in drivers/net/phy/marvell.c, but
>> > not in other source files.
>> >
>> > Hence genphy_restart_aneg() now returns 4416 instead zero, which is
>> > considered an error:
>> >
>> >     ravb e6800000.ethernet eth0: failed to connect PHY
>> >     IP-Config: Failed to open eth0
>> >     IP-Config: No network devices available
>> >
>> > Fix this by converting positive values to zero in all callers of
>> > phy_modify().
>> >
>> > Fixes: fea23fb591cce995 ("net: phy: convert read-modify-write to phy_modify()")
>> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>> > ---
>> > Alternatively, __phy_modify() could be changed to follow __phy_write()
>> > semantics?
>>
>> Hi Geert, Russell
>>
>> I took a quick look at the uses of phy_modify(). I don't see any uses
>> of the return code other than as an error indicator. So having it
>> return 0 on success seems like a better fix.
>
> I'd like to avoid that, because I don't want to have yet another
> accessor that needs to be used for advertisment modification (where
> we need to know if we changed any bits.)
>
> That's why this accessor returns the old value.

But this is documented nowhere!

I believe there are no current users of (__)phy_modify() that rely on this
behavior.  Except perhaps phy_restore_page(), which I don't understand at all.

BTW, I think phy_restore_page() may return a strict positive value as well,
thus breaking m88e1318_set_wol(), which is not supposed to return strict
positive values.

So changing __phy_modify() to return zero on success seems like the way
forward...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds