Re: [PATCH iwl-next v4 0/3] igc: add support for forcing link speed without autonegotiation

[David Laight <david.laight.linux@gmail.com>]

On Tue, 28 Apr 2026 14:00:06 +0800
KhaiWenTan <khai.wen.tan@linux.intel.com> wrote:

> From: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
> 
> This series adds support for forcing 10/100 Mb/s link speed via ethtool
> when autonegotiation is disabled on the igc driver.

I'll ask 'why' ?

In particular forcing half/full duplex has always been a very good way
of 'breaking' a network connection.

It really is much better to restrict the advertised link modes and let
the autodetect/autonegotiation logic in the phy/mac do its job.

About the only think I can think of is to force 10M HDX when connected
to a remote system that supports 10M/100M HDX.
In that case you need to send out single link test pulses, not the
burst used to identify 100M HDX, or the pattern encoded on the burst
used by autonegotiation.
But you need to got back to the mid 1990s to find such systems.
Anything that supports FDX will do autonegotiation.

	David

> 
> Changes in v4:
> - Validate that autoneg is AUTONEG_ENABLE or AUTONEG_DISABLE early
>   in igc_ethtool_set_link_ksettings() to avoid passing unexpected
>   values to igc_handle_autoneg_disabled(). (Simon Horman)
> 
> Changes in v3:
> - Modify condition from "if (duplex == DUPLEX_HALF)" to
>   "if (duplex != DUPLEX_FULL)". (Simon Horman)
> 
> Changes in v2:
> - When forcing half-duplex, set hw->fc.requested_mode = igc_fc_none,
>   since half-duplex cannot support flow control per IEEE 802.3.
>   (Simon Horman)
> - Split the original single patch into three patches for clarity:
>   patches 1 and 2 are preparatory cleanups; patch 3 carries the
>   functional change.
> 
> v3 at:
> https://patchwork.ozlabs.org/project/intel-wired-lan/cover/20260422155701.7420-1-khai.wen.tan@linux.intel.com/
> 
> v2 at:
> https://patchwork.kernel.org/project/netdevbpf/patch/20260416015520.6090-4-khai.wen.tan@linux.intel.com/
> 
> v1 at:
> https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20260409072747.217836-1-khai.wen.tan@linux.intel.com/
> 
> Faizal Rahim (3):
>   igc: remove unused autoneg_failed field
>   igc: move autoneg-enabled settings into igc_handle_autoneg_enabled()
>   igc: add support for forcing link speed without autonegotiation
> 
>  drivers/net/ethernet/intel/igc/igc_base.c    |  35 +++-
>  drivers/net/ethernet/intel/igc/igc_defines.h |   9 +-
>  drivers/net/ethernet/intel/igc/igc_ethtool.c | 209 +++++++++++++------
>  drivers/net/ethernet/intel/igc/igc_hw.h      |  10 +-
>  drivers/net/ethernet/intel/igc/igc_mac.c     |  16 +-
>  drivers/net/ethernet/intel/igc/igc_main.c    |   2 +-
>  drivers/net/ethernet/intel/igc/igc_phy.c     |  65 +++++-
>  drivers/net/ethernet/intel/igc/igc_phy.h     |   1 +
>  8 files changed, 257 insertions(+), 90 deletions(-)
> 
> --
> 2.43.0
> 
>