netdev - Re: Waiting for the PHY to complete auto-negotiation

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Date:   Thu, 7 Dec 2017 17:17:35 +0100
From:   Mason <slash.tmp@...e.fr>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>
Subject: Re: Waiting for the PHY to complete auto-negotiation

On 07/12/2017 00:00, Florian Fainelli wrote:

> On 12/06/2017 11:25 AM, Mason wrote:
>
>> When we detect link down, we put the ethernet HW block in reset,
>> and repeat initialization when the link comes back up.
>>
>> Hmmm, however, at the moment, I only reset on an administrative
>> (user-requested) link down, i.e. through ndo_stop. I would probably
>> have to handle cable unplug/replug events as well.
>>
>> Or just consider the quirk to make flow control too complicated
>> to implement correctly...
> 
> I suppose your procedure is fine, but don't you have a better way to
> resolve that by trying to place a special RX DMA ring entry that allows
> your RX DMA not to be entirely stopped, but intentionally looped through
> a buffer that you control? As long as you can stop the Ethernet MAC RX,
> working with such a limitation is probably fine, but this really sounds
> like a huge pain in the butt and a major HW flaw.

Could you elaborate a bit on your suggestion?
(Special ring entry, looped through a buffer under my control)
Is this a typical thing to do to stop DMA?

Currently the driver tries to stop DMA in nb8800_dma_stop(),
which does the following:

http://elixir.free-electrons.com/linux/latest/source/drivers/net/ethernet/aurora/nb8800.c#L881

1) poll until TX finishes (I assume the system no longer accepts new
   frames to send at this point)
2) set the EOC (end of chain) bit on all descriptors (could there be
   a problem if we receive a frame at that moment? Don't we need some
   kind of lock?)
3) disable address filtering (need to check what this does)
4) enable loop-back mode
5) send up to 5 "fake" packets in order to hit an EOC descriptor

The reason I'm trying to move away from this method is that it doesn't
work on our new SoC; and when pressed, the HW dev said it had never been
supported. (Also I find it somewhat hackish, but that's a matter of taste.)

Regards.