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

["Abdul Rahim, Faizal" <faizal.abdul.rahim@linux.intel.com>]

On 30/4/2026 10:41 pm, David Laight wrote:
> 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
> 

There's a use case requested:

Profinet Certification tool reports that forcing a link speed without
auto-negotiation is not working.
Forcing the link speed is a critical feature for the industrial automation
"fast-start" use case. When there is a connection lost, the system must
come back up as fast as possible. In PROFINET, that means to force the
speed and rejoin the controller loops. Without supporting forcing the speed
to 100M in Foxville, the certification tool would not be able to certify
the availability of this feature.

I'm hoping this context is enough to justify the need?