netdev - Re: [PATCH net-next] net: phy: sfp: handle cases where neither BR,min nor BR,max is given

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Message-ID: <20180514081151.GD24660@kwain>
Date:   Mon, 14 May 2018 10:11:51 +0200
From:   Antoine Tenart <antoine.tenart@...tlin.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com, maxime.chevallier@...tlin.com,
        gregory.clement@...tlin.com, miquel.raynal@...tlin.com,
        nadavh@...vell.com, stefanc@...vell.com, ymarkman@...vell.com,
        mw@...ihalf.com
Subject: Re: [PATCH net-next] net: phy: sfp: handle cases where neither
 BR,min nor BR,max is given

Hi Russell,

On Tue, May 08, 2018 at 01:30:26PM +0100, Russell King - ARM Linux wrote:
> 
> The electronic engineer in me says that using zero isn't really valid
> because there are coupling capacitors in the SFP module that block DC.
> These blocking capacitors are required by the SFP+ specs to have a high
> pass pole of between 20kHz and 100kHz - in other words, frequencies
> below this are attenuated by the coupling capacitors.  The relationship
> between this and the bit rate will be a function of the encoding, so we
> can't come to a definitive figure without some math (and I want to be
> lazy about that!)
> 
> Practically, we're talking about SerDes Ethernet, where the bit rate is
> no lower than 100Mbps [*], which will always have a frequency well above
> this cut-off.  So, I don't have any problem with your approach to
> setting the minimum to zero.  Therefore,
> 
> Acked-by: Russell King <rmk+kernel@...linux.org.uk>

Thanks for looking into it!
Antoine


-- 
Antoine Ténart, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com