Re: [PATCH net-next v5 4/4] phy: aquantia: Determine rate adaptation support from registers

[Sean Anderson]

On 1/5/23 12:34, Vladimir Oltean wrote:
> On Thu, Jan 05, 2023 at 11:21:14AM -0500, Sean Anderson wrote:
>> > Your code walks through the speed_table[] of media speeds (from 10M up
>> > until the max speed of the SERDES) and sees whether the PHY was
>> > provisioned, for that speed, to use PAUSE rate adaptation.
>> 
>> This is because we assume that if a phy supports rate matching for a phy
>> interface mode, then it supports rate matching to all slower speeds that
>> it otherwise supports. This seemed like a pretty reasonable assumption
>> when I wrote the code, but it turns out that some firmwares don't abide
>> by this. This is firstly a problem with the firmware (as it should be
>> configured so that Linux can use the phy's features), but we have to be
>> careful not to end up with an unsupported combination.
> 
> When you say "problem with the firmware", you're referring specifically
> to my example (10GBASE-R for >1G speeds, SGMII for <=1G speeds)?

Actually, I am mostly referring to cases where rate adaptation is set up
to use a phy interface mode which isn't supported. In particular, Tim
has a board where the phy is set up to rate adapt using 5GBASE-R (-X?),
even though the host only supports 10GBASE-R.

> Why do you consider this a firmware misconfiguration? Let's say the host
> supports both 10GBASE-R and SGMII, but the system designer preferred not
> to use PAUSE-based rate adaptation for the speeds where native rate
> adaptation was available.

This is supported, you just can't get 5G or 2.5G.

>> > If the PHY firmware uses a combination like this: 10GBASE-R/XFI for
>> > media speeds of 10G, 5G, 2.5G (rate adapted), and SGMII for 1G, 100M
>> > and 10M, a call to your implementation of
>> > aqr107_get_rate_matching(PHY_INTERFACE_MODE_10GBASER) would return
>> > RATE_MATCH_NONE, right?
>> 
>> Correct.
>> 
>> > So only ETHTOOL_LINK_MODE_10000baseT_Full_BIT
>> > would be advertised on the media side?
>> 
>> If the host only supports 10GBASE-R and nothing else. If the host
>> supports SGMII as well, we will advertise 10G, 1G, 100M, and 10M. But
>> really, this is a problem with the firmware, since if the host supports
>> only 10GBASE-R, then the firmware should be set up to rate adapt to all
>> speeds.
> 
> So we lose the advertisement of 5G and 2.5G, even if the firmware is
> provisioned for them via 10GBASE-R rate adaptation, right? Because when
> asked "What kind of rate matching is supported for 10GBASE-R?", the
> Aquantia driver will respond "None".

Correct.

>> > Shouldn't you take into consideration in your aqr107_rate_adapt_ok()
>> > function only the media side link speeds for which the PHY was actually
>> > *configured* to use the SERDES protocol @iface?
>> 
>> No, because we don't know what phy interface modes are actually going to
>> be used. The phy doesn't know whether e.g. the host supports both
>> 10GBASE-R and SGMII or whether it only supports 10GBASE-R. With the
>> current API we cannot say "I support 5G" without also saying "I support
>> 1G". If you don't like this, please send a patch for an API returning
>> supported speeds for a phy interface mode.

Again, this is to comply with the existing API assumptions. The current
code is buggy. Of course, another way around this is to modify the API.
I have chosen this route because I don't have a situation like you
described. But if support for that is important to you, I encourage you
to refactor things.

--Sean