netdev - Re: [PATCH net-next v2 2/2] net: phy: marvell: add PHY tunable fast link down support for 88E1540

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Date:   Mon, 25 Mar 2019 20:13:30 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        David Miller <davem@...emloft.net>,
        "John W. Linville" <linville@...driver.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 2/2] net: phy: marvell: add PHY tunable fast
 link down support for 88E1540

On 25.03.2019 19:44, Florian Fainelli wrote:
> On 3/25/19 11:35 AM, Heiner Kallweit wrote:
>> 1000BaseT standard requires that a link is reported as down earliest
>> after 750ms. Several use case however require a much faster detecion
>> of a broken link. Fast Link Down supports this by intentionally
>> violating a the standard. This patch exposes the Fast Link Down
>> feature of 88E1540 and 88E6390. These PHY's can be found as internal
>> PHY's in several switches: 88E6352, 88E6240, 88E6176, 88E6172,
>> and 88E6390(X). Fast Link Down and EEE are mutually exclusive.
>>
>> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
> 
> This looks fine, just one question though: do not you need to verify
> that the phy_interface_t maps to a copper medium somehow?
> 
Good question. The register we configure is used in copper mode
only (see name: COPPER_CTRL3). So we don't cause any harm at least
if we configure Fast Link Down when being, let's say, in a fiber mode.
Fast Link Down is applicable for 1000BaseT only, so in theory what
you say would apply also to an attempt to configure Fast Link Down
when being in 100BaseT or 10GBaseT copper modes.
Being allowed to configure a feature for 1000BaseT whilst being in
a different mode can been seen as a feature (what I do) or as a bug.
Most likely there is no 100% answer to this question.