Re: [net-next PATCH v1 0/2] stmmac: dwmac-meson8b: configurable RGMII TX delay

[Florian Fainelli <f.fainelli-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 11/25/2016 03:13 AM, Sebastian Frias wrote:
> 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-rdvid1DuHRBWk0Htik3J/w@public.gmane.org> 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.

The placement of the (delay) was not meant to be exact, but it was
wrongly place anyway, so it should be between the MAC and PHY, always.
This is why you see people either fixing the need for a delay by
appropriately programming the PHY, or the MAC, or by just inserting a
fixed delay on the PCB between the PHY and the MAC and programming no
delays (or using the default values and hoping this works).

> 
>>> 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?

That's one possibility, although, see below, some PHYs support
sub-nanosecond values, but in general, that seems like a good
representation. If the "txid-delay-ns" property is omitted, a standard
2ns delay is assumed.

> 
>> 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?

Yes, indeed.

> 
> 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?

I doubt this exists at the MAC level what we should have is either a 2ns
delay, in either RX or TX path, or nothing, because that's the value
that results in shifting the data lines and the RX/TX lines by 90
degrees at 125Mhz (1/125^6 = 8 ns, one quarter shift is 90 degrees =
2ns). The PHY may have a similar set of pre-programmed, fixed 2ns
delays, but it is not uncommon to see 0.X ns resolution available:

drivers/net/phy/mscc.c
drivers/net/phy/dp83867.c w/ arch/arm/boot/dts/dra72-evm-revc.dts

In these cases, if you end-up using a non 2ns delay, you are fixing a
PCB problem more than an interoperability problem between your MAC and PHY.

> 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 suppose such a thing would work yes, but it would be difficult to
report correctly to the core PHYLIB how this can work considering the
vast array of options available to introduce delays in that case:
MAC-level, PHY-level, pinctrl/pad level and possibly at the PCB itself.

Once we can't rely on the fixed 2ns delay to work, we are going to have
people do various experiments until they can either measure what the
right delay value is for the specific PCB, or they just found the value
that happens to work. I don't think we can do much at that point from a
core PHYLIB perspective other than tell the network driver that the PHY
supports delay in either RX, TX or both directions, and have the MAC
decide what to apply that makes sense here, considering that this is
already kind of an exceptional situation to be in.
-- 
Florian
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