linux-kernel - Re: [RFC 01/11] net: phy: Don't report success if devices weren't found

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Date:   Sun, 24 May 2020 21:46:55 -0500
From:   Jeremy Linton <jeremy.linton@....com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, andrew@...n.ch,
        f.fainelli@...il.com, hkallweit1@...il.com,
        madalin.bucur@....nxp.com, calvin.johnson@....nxp.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 01/11] net: phy: Don't report success if devices weren't
 found

Hi,

Thanks for taking a look at this.

On 5/23/20 1:20 PM, Russell King - ARM Linux admin wrote:
> On Fri, May 22, 2020 at 04:30:49PM -0500, Jeremy Linton wrote:
>> C45 devices are to return 0 for registers they haven't
>> implemented. This means in theory we can terminate the
>> device search loop without finding any MMDs. In that
>> case we want to immediately return indicating that
>> nothing was found rather than continuing to probe
>> and falling into the success state at the bottom.
> 
> This is a little confusing when you read this comment:
> 
>                          /*  If mostly Fs, there is no device there,
>                           *  then let's continue to probe more, as some
>                           *  10G PHYs have zero Devices In package,
>                           *  e.g. Cortina CS4315/CS4340 PHY.
>                           */
> 
> Since it appears to be talking about the case of a PHY where *devs will
> be zero.  However, tracking down the original submission, it seems this
> is not the case at all, and the comment is grossly misleading.
> 
> It seems these PHYs only report a valid data in the Devices In Package
> registers for devad=0 - it has nothing to do with a zero Devices In
> Package.

Yes, this ended up being my understanding of this commit, and is part of 
my justification for starting the devices search at the reserved address 
0 rather than 1.

> 
> Can I suggest that this comment is fixed while we're changing the code
> to explicitly reject this "zero Devices In package" so that it's not
> confusing?

Its probably better to kill it in 5/11 with a mention that we are 
starting at a reserved address?

OTOH, I'm a bit concerned that reading at 0 as the first address will 
cause problems because the original code was only triggering it after a 
read returning 0xFFFFFFFF at a valid MMD address. It does 
simplify/clarify the loop though. If it weren't for this 0 read, I would 
have tried to avoid some of the additional MMD reserved addresses.