Problem with "swinging" ethernet link on i.MX28 based device

[Michael Heimpold <michael.heimpold@i2se.com>]

Hi,

we at I2SE have developed an i.MX28 based embedded device, called Duckbill.
Regarding ethernet, it is very similar to the FSL mx28evk, i.e. we use the same
phy (SMSC LAN8720), we use RMII mode, clock is provided by iMX (not via
external crystal), we have a GPIO to reset the phy and an interrupt GPIO.

At the moment, we are using vanilla kernel 4.2.5 with only a fetch patches - mainly
patches to add the device tree stuff needed and some cherry-picked upstream
patches. Nothing special IMO. You can find it here:
https://github.com/I2SE/linux/tree/v4.2.5-duckbill

After booting the device, we observe on a few devices the ethernet "swinging"
or toggling:
[   15.830759] fec 800f0000.ethernet eth0: Freescale FEC PHY driver [SMSC LAN8710/LAN8720] (mii_bus:phy_addr=800f0000.etherne:00, irq=35)
[   15.843361] IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready
[   17.112867] fec 800f0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
[   17.121746] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[   17.894416] fec 800f0000.ethernet eth0: Link is Down
[   19.506960] fec 800f0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
[   20.294398] fec 800f0000.ethernet eth0: Link is Down
[   21.932365] fec 800f0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
[   22.714410] fec 800f0000.ethernet eth0: Link is Down
[   24.357890] fec 800f0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
[   25.144400] fec 800f0000.ethernet eth0: Link is Down
[   26.783367] fec 800f0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
[   27.564357] fec 800f0000.ethernet eth0: Link is Down
[   29.208846] fec 800f0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx

However, in U-Boot I can transfer several hundreds of MB without any problem,
I can attach the cable/reattach etc. as usual - no obvious problems.
This is the reason, we think that the hardware is ok (and of course, we double-checked
especially the soldering of the affected devices).

This is what I tried so far:
- changed the device tree to not use the interrupt line -> phy is polled periodically
- reverted latest phy changes for LAN8720, i.e.
   d88ecb373bd1877acc43e13311a8e0e6daffc3d2 - "net: phy: smsc: disable energy detect mode"
   ff94c742dfeea3110f1e1d27399d728f8494d29e - "net: phy: fix semicolon.cocci warnings"
   776829de90c5972895db398993ddfa9417ff8b01 - "net: phy: workaround for buggy cable detection by LAN8700 after cable plugging"
- then I tried the following patch

-snip-
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -936,7 +936,7 @@ fec_restart(struct net_device *ndev)
         if (fep->quirks & FEC_QUIRK_HAS_AVB) {
                 writel(0, fep->hwp + FEC_ECNTRL);
         } else {
- writel(1, fep->hwp + FEC_ECNTRL);
+ //writel(1, fep->hwp + FEC_ECNTRL);
                 udelay(10);
         }
-snap-
The idea behind this patch is: when FEC is resetted, then registers are reset to initial values
and thus FEC is configured for MII. When using ENET_CLK_OUT this results in 25 MHz clock
to phy instead of 50 MHz as required by RMII. At least for a short time, there might be
a "glitch". The concern was, that the phy might become stuck by this.

However, nothing of the above improved anything. So, at the moment, I'm
running out of ideas - any help/hint to debug this issue further is really appreciated.
Please let me know if I missed some important information.

Mit freundlichen Grüßen / Kind regards
Michael Heimpold
-- 
Software Engineer

I2SE GmbH                           Tel: +49 (0) 341 355667-00
Friedrich-Ebert-Str. 61             Fax: +49 (0) 341 355667-02
04109 Leipzig
Germany
Web: http://www.i2se.com/           Mail: info@i2se.com
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Amtsgericht Leipzig HRB 23784
Geschäftsführer/CEO: Carsten Ziermann

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