[U-Boot] [PATCH V7] ARM: mx6: Add support for Kosagi Novena

[Nikolay Dimitrov]

Hi Marek,

On 10/15/2014 12:38 AM, Marek Vasut wrote:
> On Sunday, October 12, 2014 at 08:33:21 AM, Sean Cross wrote:
>> On 12/10/2014 05:04, Fabio Estevam wrote:
>>> On Sat, Oct 11, 2014 at 11:21 AM, Sean Cross <xobs at kosagi.com> wrote:
>>>>> Ok, understood. Just curious: which Ethernet PHY is used on the novena
>>>>> board?
>>>>
>>>> It's the same Micrel PHY used on the Sabrelite, the KSZ9021.
>>>
>>> nitrogen/sabrelite holds Ethernet PHY reset low for 10ms, which is in
>>> accordance with ksz9021 datasheet.
>>>
>>> Shouldn't we wait 10ms here as well?
>>
>> The reference manual for the PHY indicates that you should hold reset
>> low for 10ms after the supply voltage stabilizes.  So long as it takes
>> at least 10msto get from the point at which the CPU starts executing its
>> ROM code  to the point at which the reset line is toggled, we will be fine.
>
> This definitelly is the case, so I presume we don't need the delay ?

Well, here's how I see the case.

After power on, the PHY unfortunately is out of reset (R20G is DNP, imx6 
pin pulled high internally after reset). At some unknown point in time 
the CPU reaches novena_spl_setup_iomux_enet(). During all this time the 
PHY is still out of reset. Neither this complies with the recommended 
sequence, and even more doesn't comply if we remove the delay.

If we leave the delay as it is currently implemented, the actual reset 
sequence is just delayed (by the time it takes the CPU to reach the PHY 
reset code). At this later point we enforce the proper reset sequence: 
supply rail is obviously now stable, and we keep the PHY reset low for 
the minimum specified time: 10ms.

My understanding is that this is simple and efficient way to guarantee 
that for all different cases (different temperatures, different CPU 
silicon revisions, differently configured drivers/subsystems), the PHY 
reset timing is generated properly, and will be generated properly in 
the future when the code evolves.

Please tell me if I'm missing something.

Kind regards,
Nikolay