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

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

[Sven Eckelmann]

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.

Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
 compat-include/linux/ethtool.h |  2 --
 net/batman-adv/bat_v_elp.c     | 15 +--------------
 2 files changed, 1 insertion(+), 16 deletions(-)

diff --git a/compat-include/linux/ethtool.h b/compat-include/linux/ethtool.h
index e1f39c33..8dcbe02c 100644
--- a/compat-include/linux/ethtool.h
+++ b/compat-include/linux/ethtool.h
@@ -21,7 +21,6 @@ struct batadv_ethtool_link_ksettings {
 	struct {
 		__u32	speed;
 		__u8	duplex;
-		__u8	autoneg;
 	} base;
 };
 
@@ -42,7 +41,6 @@ batadv_ethtool_get_link_ksettings(struct net_device *dev,
 		return ret;
 
 	link_ksettings->base.duplex = cmd.duplex;
-	link_ksettings->base.autoneg = cmd.autoneg;
 	link_ksettings->base.speed = ethtool_cmd_speed(&cmd);
 
 	return 0;
diff --git a/net/batman-adv/bat_v_elp.c b/net/batman-adv/bat_v_elp.c
index 2614a9ca..a39af0ee 100644
--- a/net/batman-adv/bat_v_elp.c
+++ b/net/batman-adv/bat_v_elp.c
@@ -120,20 +120,7 @@ static u32 batadv_v_elp_get_throughput(struct batadv_hardif_neigh_node *neigh)
 	rtnl_lock();
 	ret = __ethtool_get_link_ksettings(hard_iface->net_dev, &link_settings);
 	rtnl_unlock();
-
-	/* Virtual interface drivers such as tun / tap interfaces, VLAN, etc
-	 * tend to initialize the interface throughput with some value for the
-	 * sake of having a throughput number to export via ethtool. This
-	 * exported throughput leaves batman-adv to conclude the interface
-	 * throughput is genuine (reflecting reality), thus no measurements
-	 * are necessary.
-	 *
-	 * Based on the observation that those interface types also tend to set
-	 * the link auto-negotiation to 'off', batman-adv shall check this
-	 * setting to differentiate between genuine link throughput information
-	 * and placeholders installed by virtual interfaces.
-	 */
-	if (ret == 0 && link_settings.base.autoneg == AUTONEG_ENABLE) {
+	if (ret == 0) {
 		/* link characteristics might change over time */
 		if (link_settings.base.duplex == DUPLEX_FULL)
 			hard_iface->bat_v.flags |= BATADV_FULL_DUPLEX;
-- 
2.20.1

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

[Marek Lindner]

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

[Sven Eckelmann]

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On Monday, 25 November 2019 11:02:17 CET Marek Lindner wrote:
[...]
> > 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.

It is also not reliable with auto negotiation. And disabled auto negotiation
is also used rather often in complete valid setups with fixed links.

And I never said "better" - I said "more". And the other cases can be solved 
the same way as described in the original patch - override them.

> 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.

Routing loops? Are you sure? If this is the case than we have a major flaw in 
B.A.T.M.A.N. V and should think about disabling it again. This should never 
happen - because bad measurements can always happen. And who says that it is
not a common case (even without ethernet).... maybe because it will be rather 
normal in the wild due to the various intepretation what 
get_expected_throughput should represent. Every non-minstrel based driver 
seems to have its own idea of what get_expected_throughput should return.

The only thing I am aware of are the invalid looking throughput values. But as 
outlined before, ethtool's link_ksetting are a rather bad source for neighbor 
specific throughput measurements. But it is the only thing which we have 
without a neighbor specific measurement for neighbors.

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

"auto negotiation has nothing to do with the validity of the retrieved
values"

And gluon users/developers noticed that a lot of ethtool's link_ksetting
were not used because of this patch.

Another option would be to completely drop the usage of ethtool's
link_ksettings without having automatic measurements. But I think no
one will be happy with it.

Kind regards,
	Sven

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