netdev - Re: [PATCH net-next 1/2] mlxsw: core: Add ethtool support for QSFP-DD transceivers

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Date:   Tue, 30 Jun 2020 08:59:45 +0300
From:   Ido Schimmel <idosch@...sch.org>
To:     Andrew Lunn <andrew@...n.ch>, vadimp@...lanox.com
Cc:     Adrian Pop <popadrian1996@...il.com>, netdev@...r.kernel.org,
        davem@...emloft.net, kuba@...nel.org, jiri@...lanox.com,
        mlxsw@...lanox.com, Ido Schimmel <idosch@...lanox.com>
Subject: Re: [PATCH net-next 1/2] mlxsw: core: Add ethtool support for
 QSFP-DD transceivers

On Tue, Jun 30, 2020 at 02:21:59AM +0200, Andrew Lunn wrote:
> I've no practice experience with modules other than plain old SFPs,
> 1G. And those have all sorts of errors, even basic things like the CRC
> are systematically incorrect because they are not recalculated after
> adding the serial number. We have had people trying to submit patches
> to ethtool to make it ignore bits so that it dumps more information,
> because the manufacturer failed to set the correct bits, etc.
> 
> Ido, Adrian, what is your experience with these QSFP-DD devices. Are
> they generally of better quality, the EEPROM can be trusted? Is there
> any form of compliance test.

Vadim, I know you tested with at least two different QSFP-DD modules,
can you please share your experience?

> 
> If we go down the path of using the discovery information, it means we
> have no way for user space to try to correct for when the information
> is incorrect. It cannot request specific pages. So maybe we should
> consider an alternative?
> 
> The netlink ethtool gives us more flexibility. How about we make a new
> API where user space can request any pages it want, and specify the
> size of the page. ethtool can start out by reading page 0. That should
> allow it to identify the basic class of device. It can then request
> additional pages as needed.

Just to make sure I understand, this also means adding a new API towards
drivers, right? So that they only read from HW the requested info.

> The nice thing about that is we don't need two parsers of the
> discovery information, one in user and second in kernel space. We
> don't need to guarantee these two parsers agree with each other, in
> order to correctly decode what the kernel sent to user space. And user
> space has the flexibility to work around known issues when
> manufactures get their EEPROM wrong.

Sounds sane to me... I know that in the past Vadim had to deal with
various faulty modules. Vadim, is this something we can support? What
happens if user space requests a page that does not exist? For example,
in the case of QSFP-DD, lets say we do not provide page 03h but user
space still wants it because it believes manufacturer did not set
correct bits.