Message-ID: <6a6af561-4e83-ca6e-d989-f421e18bce1e@laposte.net>
Date:   Fri, 25 Nov 2016 12:13:32 +0100
From:   Sebastian Frias <sf84@...oste.net>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Jerome Brunet <jbrunet@...libre.com>
Cc:     linux-amlogic@...ts.infradead.org, devicetree@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, khilman@...libre.com,
        mark.rutland@....com, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, alexandre.torgue@...com,
        peppe.cavallaro@...com, carlo@...one.org,
        Mans Rullgard <mans@...sr.com>, Andrew Lunn <andrew@...n.ch>
Subject: Re: [net-next PATCH v1 0/2] stmmac: dwmac-meson8b: configurable RGMII
 TX delay

On 24/11/16 19:55, Florian Fainelli wrote:
> Le 24/11/2016 à 09:05, Martin Blumenstingl a écrit :
>> On Thu, Nov 24, 2016 at 4:56 PM, Jerome Brunet <jbrunet@...libre.com> wrote:
>>> On Thu, 2016-11-24 at 15:34 +0100, Martin Blumenstingl wrote:
>>>> Currently the dwmac-meson8b stmmac glue driver uses a hardcoded 1/4
>>>> cycle TX clock delay. This seems to work fine for many boards (for
>>>> example Odroid-C2 or Amlogic's reference boards) but there are some
>>>> others where TX traffic is simply broken.
>>>> There are probably multiple reasons why it's working on some boards
>>>> while it's broken on others:
>>>> - some of Amlogic's reference boards are using a Micrel PHY
>>>> - hardware circuit design
>>>> - maybe more...
>>>>
>>>> This raises a question though:
>>>> Which device is supposed to enable the TX delay when both MAC and PHY
>>>> support it? And should we implement it for each PHY / MAC separately
>>>> or should we think about a more generic solution (currently it's not
>>>> possible to disable the TX delay generated by the RTL8211F PHY via
>>>> devicetree when using phy-mode "rgmii")?
>>>
>>> Actually you can skip the part which activate the Tx-delay on the phy
>>> by setting "phy-mode = "rgmii-id" instead of "rgmii"
>>>
>>> phy->interface will no longer be PHY_INTERFACE_MODE_RGMII
>>> but PHY_INTERFACE_MODE_RGMII_ID.
>> unfortunately this is not true for RTL8211F (I did my previous tests
>> with the same expectation in mind)!
>> the code seems to suggest that TX-delay is disabled whenever mode !=
>> PHY_INTERFACE_MODE_RGMII.
>> BUT: on my device RTL8211F_TX_DELAY is set even before
>> "phy_write(phydev, 0x11, reg);"!

If you look at the Atheros 803x PHY and its driver
'drivers/net/phy/at803x.c':
- by default (as HW reset preset) the PHY has RX delay enabled, TX
delay disabled
- the driver only enables RX, or TX, or both, according to "rgmii-rxid",
"rgmii-txid", or "rgmii-id" respectively, but does not alter HW reset
presets. In other words:
  a "rgmii-rxid" results in RX enabled (expected)
  b "rgmii-txid" results in RX *and* TX enabled (unexpected?)
  c "rgmii-id"   results in RX *and* TX enabled (expected)
  d "rgmii"      results in RX enabled (unexpected?)

This is a bit surprising and I think that some boards and PHY<->MAC
combinations are working a little bit by chance, unless I'm missing
something.

> 
> (Adding Sebastian (and Mans, and Andrew) since he raised the same
> question a while ago. I think I now understand a bit better what
> Sebastian was after a couple of weeks ago)
> 

Thanks for CCing us, it is indeed a very similar issue.

>>
>> Based on what I found it seems that rgmii-id, rgmii-txid and
>> rgmii-rxid are supposed to be handled by the PHY.
> 
> Correct, the meaning of PHY_INTERFACE_MODE should be from the
> perspective of the PHY device:
> 
> - PHY_INTERFACE_MODE_RGMII_TXID means that the PHY is responsible for
> adding a delay when the MAC transmits (TX MAC -> PHY (delay) -> wire)
> - PHY_INTERFACE_MODE_RGMII_RXID means that the PHY is responsible for
> adding a delay when the MAC receives (RX MAC <- (delay) PHY) <- wire)
> 

Thanks for the explanation.
Actually I had thought that the delay was to account for board routing
(wires) between the MAC and the PHY.
>From your explanation it appears that the delay is to account for board
routing (wires) between the PHY and the RJ45 socket.

>> That would mean that we have two problems here:
>> 1) drivers/net/phy/realtek.c:rtl8211f_config_init should check for
>> PHY_INTERFACE_MODE_RGMII_ID or PHY_INTERFACE_MODE_RGMII_TXID and
>> enable the TX-delay in that case - otherwise explicitly disable it
> 
> Agreed.
> 
>> 2) dwmac-meson8b.c should only use the configured TX-delay for
>> PHY_INTERFACE_MODE_RGMII
>> @Florian: could you please share your thoughts on this (who handles
>> the TX delay in which case)?
> 
> This also seems reasonable to do, provided that the PHY is also properly
> configured not to add delays in both directions, and therefore assumes
> that the MAC does it.
> 
> We have a fairly large problem with how RGMII delays are done in PHYLIB
> and Ethernet MAC drivers (or just in general), where we can't really
> intersect properly what a PHY is supporting (in terms of internal
> delays), and what the MAC supports either. One possible approach could
> be to update PHY drivers a list of PHY_INTERFACE_MODE_* that they
> support (ideally, even with normalized nanosecond delay values), 

Just to make sure I understood this, the DT would say something like:

phy-connection-type = "rgmii-txid";
txid-delay-ns = <3>;

For a 3ns TX delay, would that be good?

>and
> then intersect that with the requested phy_interface_t during
> phy_{attach,connect} time, and feed this back to the MAC with a special
> error code/callback, so we could gracefully try to choose another
> PHY_INTERFACE_MODE_* value that the MAC supports....
> 
> A larger problem is that a number of drivers have been deployed, and
> Device Trees, possibly with the meaning of "phy-mode" and
> "phy-connection-type" being from the MAC perspective, and not the PHY
> perspective *sigh*, good luck auditing those.
> 
> So from there, here is possibly what we could do
> 
> - submit a series of patches that update the PHYLIB documentation (there
> are other things missing here) and make it clear from which entity (PHY
> or MAC) does the delay apply to, document the "intersection" problem here

I think documenting is necessary, thanks in advance!

However, I'm wondering if there's a way to make this work in all cases.
Indeed, if we consider for example that TX delay is required, we have 4
cases:

       PHY         |       MAC          | Who applies?
TXID supported     | TXID supported     | PHY
TXID supported     | TXID not supported | PHY
TXID not supported | TXID supported     | MAC
TXID not supported | TXID not supported | cannot be done

That is basically what my patch:

https://marc.info/?l=linux-netdev&m=147869658031783&w=2

attempted to achieve. That would allow more combinations of MAC<->PHY to
work, right?

Nevertheless, I think we also need to keep in mind that most of this
discussion assumes the case where both, MAC and PHY have equal capabilities.
Could it happen that the PHY supports only 2ns delay, and the MAC only
1ns delay?
Could it happen that the delay is bigger than what is supported by
either the PHY or MAC alone? maybe if combined it could work, for example
a 3ns delay required and the PHY supporting 2ns and the MAC 1ns, combined
it could work?

I don't know if these are cases worth supporting, nor if they are valid.

> 
> - have you document the configured behavior for dwmac-meson8b that we
> just discussed here in v2 of this patch series
> 
> Sorry for the long post, here is a virtual potato: 0
> 

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