Re: [PATCH] batman-adv: Revert "disable ethtool link speed detection when auto negotiation off"

[Marek Lindner <mareklindner@neomailbox.ch>]

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On Monday, 25 November 2019 17:46:50 HKT Sven Eckelmann wrote:
> The commit d60b8fc69ef2 ("batman-adv: disable ethtool link speed detection
> when auto negotiation off") disabled the usage of ethtool's link_ksetting
> when auto negotation was enabled due to invalid values when used with
> tun/tap virtual net_devices. According to the patch, automatic measurements
> should be used for these kind of interfaces.
> 
> But there are major flaws with this argumentation:
> 
> * automatic measurements are not implemented
> * auto negotiation has nothing to do with the validity of the retrieved
>   values
> 
> The first point has to be fixed by a longer patch series. The "validity"
> part of the second point must be addressed in the same patch series by
> dropping the usage of ethtool's link_ksetting (thus always doing automatic
> measurements over ethernet).
> 
> Drop the patch again to have more default values for various net_device
> types/configurations. The user can still overwrite them using the
> batadv_hardif's BATADV_ATTR_THROUGHPUT_OVERRIDE.

I am not quite clear on how reverting this patch will get us better default 
values. In the case reported by Matthias the autoneg detection was working as 
intended by this very patch you are reverting. As Antonio had originally 
outlined:

The problem with autonegotiation disabled is that the advertised speed
is likely to be a random number set by default by the driver.

This patch was the main reason why Matthias (or Gluon users) even realized 
that there was an issue with certain Ethernet ports & BATMAN V. Without the 
patch BATMAN V may have created routing loops and Gluon users would complain 
about those instead.

There is no disagreement that the situation needs improving but why is 
reverting this autoneg patch the best course of action ?

Cheers,
Marek

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