Re: [PATCH] net: phy: at803x: disable delay only for RGMII mode

[Florian Fainelli <f.fainelli@gmail.com>]

On 2/13/19 9:40 AM, Niklas Cassel wrote:
> On Wed, Feb 13, 2019 at 02:40:18PM +0100, Marc Gonzalez wrote:
>> On 13/02/2019 14:29, Andrew Lunn wrote:
>>
>>>> So we have these modes:
>>>>
>>>> PHY_INTERFACE_MODE_RGMII: TX and RX delays disabled
>>>> PHY_INTERFACE_MODE_RGMII_ID: TX and RX delays enabled
>>>> PHY_INTERFACE_MODE_RGMII_RXID: RX delay enabled, TX delay disabled
>>>> PHY_INTERFACE_MODE_RGMII_TXID: TX delay enabled, RX delay disabled
>>>>
>>>> What I don't like with this patch, is that if we specify phy-mode
>>>> PHY_INTERFACE_MODE_RGMII_TXID, this patch will enable TX delay,
>>>> but RX delay will not be explicitly set.
>>>
>>> That is not the behaviour we want. It is best to assume the device is
>>> in a random state, and correctly enable/disable all delays as
>>> requested. Only leave the hardware alone if PHY_INTERFACE_MODE_NA is
>>> used.
>>
>> That's what my patch did:
>> https://www.spinics.net/lists/netdev/msg445053.html
>>
>> But see Florian's remarks:
>> https://www.spinics.net/lists/netdev/msg445133.html
> 
> Hello Marc,
> 
> I saw that comment from Florian. However that was way back in 2017.
> Maybe the phy-modes were not as well defined back then?

The definition of the 'phy-mode' was clarified to be understood from the
perspective of the PHY device (hence the name) after we had several
fruitful exchanges with Marc (at least from my perspective), but since
the definition was not clear before, there is a high chance of finding
DTS/DTBs out there with the 'phy-mode' property understood from the
MAC's perspective, which would now be wrong.


> 
> Andrew recently suggested to fix the driver so that it conforms with the
> phy-modes, and fix any SoC that specified an incorrect phy-mode in DT
> and thus relied upon the broken behavior of the PHY driver:
> https://www.spinics.net/lists/netdev/msg445133.html
> 
> 
> So, I've rebased your old patch, see attachment.
> I suggest that Peter test it on am335x-evm.
> 
> am335x-evm appears to rely on the current broken behavior of the PHY
> driver, so we will probably need to fix the am335x-evm according to this:
> https://www.spinics.net/lists/netdev/msg445117.html
> and merge that as well.
> 
> 
> Andrew, Florian, do you both agree?

In my reply to Marc, there was a concern that while am335x-evm was
identified and reported to be broken after fixing the PHY driver, there
could be platforms out there that we have little to no visibility that
would most likely be equally broken. That concern still exists, and I
don't think there is anything we can do to even assess the size of the
problem unless we attempt to fix it, so maybe we should attempt to fix that.

There was a suggestion to Marc that one way to possibly "ignore" an
incorrectly broken 'phy-mode' property would be to allow specifying
rx/tx delay properties such that if the driver obtained its
phy_interface_t, yet still parsed rx/tx delays, the rx/tx delays would
take precedence, and we could possibly derive some sort of a "more
correct" phy_interface_t that we could assign back to phydev->interface
and issue a warning about that.

Another possible way to resolve that could be to introduce a 'mac-mode'
property, which must be strictly compatible with specifying a 'phy-mode'
property. For instance:

- MAC specifies mac-mode = 'rgmii-id', then the PHY must have phy-mode =
'rmgii' since the MAC is taking of inserting both RX and TX delays,
reverse also applies

- MAC specifies mac-mode = 'rgmii-txid', then the PHY must have phy-mode
= 'rgmii-rxid' because the MAC adds the TX delay, but the PHY should
insert the delay on the RX lines, reverse also applies

Because there is usually (not always, DSA is an exception) a 1:1 mapping
between MAC and PHY devices we could look up the 'mac-mode' property in
the MAC in the PHY library code and make sure that we have a compatible
matrix and if we do not, maybe pass something like PHY_INTERFACE_MODE_NA
such that the driver retains its settings.

Maybe another way to approach this is if we assume that the PHY comes up
configured correctly by the boot loader, or upon power on reset, we add
some PHY driver methods that allow us to determine the RGMII mode in
which a PHY is and that tells us whether we are compatible with the
MAC's phy_interface_t upon connection. We check both at connect() time
and if something does not look right, we flip the meaning of
phy_interface_t.

None of those solutions are entirely fool proof, but at least we might
be able to detect incorrect combinations, yet still make them work by
reversing the meaning of the 'phy-mode' property given information at hand.

Let me know if none of that makes sense and this just looks like yet
another brain dump.

Wonderful RGMII...
-- 
Florian