[GIT PULL] Allwinner Fixes for 5.10

[Ard Biesheuvel]

On Fri, 30 Oct 2020 at 10:06, Arnd Bergmann <arnd at kernel.org> wrote:
>
> On Thu, Oct 29, 2020 at 11:59 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> >
> > This is what Ilias and I tried to argue [0] earlier today, but Andrew
> > has a different view.
> >
> > [0] https://lore.kernel.org/netdev/CAMj1kXGQDeOGj+2+tMnPhjoPJRX+eTh8-94yaH_bGwDATL7pkg@mail.gmail.com/
>
> Could we work around it in by updating the DT contents at boot time before it
> gets to the driver? We've done that in the past on specific machines to avoid
> breaking DT compatibility, though I don't like the idea of doing it
> for everyone.
>
> Is there a safe fallback? I.e. would it break other platforms if we were to
> always replace phy-mode="rgmii" with phy-mode="rgmii-id"? I suppose we
> have no way of detecting the actual phy from early DT code in order to
> figure out if the device behind the mdio bus is affected. The best we could
> do might be either a list of affected boards or a list of affected ethernet
> controllers for which this gets updated.
>

The root of the issue is that the followup fix that was backported
overrides the TX/RX delay setting configured by h/w straps.

So 'rgmii' used to mean 'don't enable TX/RX delay in software' but now
it means 'disable TX/RX delay in software even if it was enabled using
h/w straps'.

Omitting the phy-mode string entirely would help, as the PHY layer
interprets this as 'leave the delay setting alone'. However, there are
cases where the MAC driver may interpret this property as well, as it
may have to apply the delay at the MAC end if the PHY is programmed
for no delay. Also, it may need to distinguish between gmii and rgmii.

So we cannot replace rgmii with anything else in the general case, the
only thing we can do is leave the TX/RX delay setting alone if we
detect that it is enabled by h/w straps. According to Andrew, this is
possible, and he is willing to accept a patch that implements this for
backporting to -stable, but otherwise treats it as someone else's
problem.