On Sat, 2011-04-16 at 14:06 +0200, Fejes József wrote:
[...]
> I took a deeper look. It seems to me that the firmware files are not the 
> usual microcode type that the device can't function without, it just 
> sets up some registers, which supposedly already contain some sensible 
> values, so it's more like patching.

Some of the R8169 variants have a microcontroller in the PHY running
firmware that is initially loaded from non-volatile memory (maybe
eFuse?).  These blobs contain bug fixes for the original PHY firmware.

> That explains why this device still 
> works without the firmware. So my actual question is this: what do I 
> gain if I use the firmware, what do I lose if I don't?
[...]

The original firmware apparently is unable to establish a stable link
against some link partners.

This warning:

> W: Possible missing firmware /lib/firmware/rtl_nic/rtl8168d-2.fw for 
> module r8169
> W: Possible missing firmware /lib/firmware/rtl_nic/rtl8168d-1.fw for 
> module r8169

is purely based on the MODULE_FIRMWARE annotations, which do not
distinguish which devices might require which files.

Ben.

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.