Re: [PATCH net-next v2] net: phy: dp83867: Add speed optimization feature

[Dan Murphy]

Heiner

On 2/6/20 4:28 PM, Heiner Kallweit wrote:
> On 06.02.2020 23:13, Dan Murphy wrote:
> > Heiner
> >
> > On 2/5/20 3:16 PM, Heiner Kallweit wrote:
> > > On 04.02.2020 19:13, Dan Murphy wrote:
> > > > Set the speed optimization bit on the DP83867 PHY.
> > > > This feature can also be strapped on the 64 pin PHY devices
> > > > but the 48 pin devices do not have the strap pin available to enable
> > > > this feature in the hardware. PHY team suggests to have this bit set.
> > > >
> > > > With this bit set the PHY will auto negotiate and report the link
> > > > parameters in the PHYSTS register. This register provides a single
> > > > location within the register set for quick access to commonly accessed
> > > > information.
> > > >
> > > > In this case when auto negotiation is on the PHY core reads the bits
> > > > that have been configured or if auto negotiation is off the PHY core
> > > > reads the BMCR register and sets the phydev parameters accordingly.
> > > >
> > > > This Giga bit PHY can throttle the speed to 100Mbps or 10Mbps to accomodate a
> > > > 4-wire cable. If this should occur the PHYSTS register contains the
> > > > current negotiated speed and duplex mode.
> > > >
> > > > In overriding the genphy_read_status the dp83867_read_status will do a
> > > > genphy_read_status to setup the LP and pause bits. And then the PHYSTS
> > > > register is read and the phydev speed and duplex mode settings are
> > > > updated.
> > > >
> > > > Signed-off-by: Dan Murphy <dmurphy@xxxxxx>
> > > > ---
> > > > v2 - Updated read status to call genphy_read_status first, added link_change
> > > > callback to notify of speed change and use phy_set_bits - https://lore.kernel.org/patchwork/patch/1188348/
> > > As stated in the first review, it would be appreciated if you implement
> > > also the downshift tunable. This could be a separate patch in this series.
> > > Most of the implementation would be boilerplate code.
> >
> > I looked at this today and there are no registers that allow tuning the downshift attempts. There is only a RO register that tells you how many attempts it took to achieve a link. So at the very least we could put in the get_tunable but there will be no set.
> The get operation for the downshift tunable should return after how many failed
> attempts the PHY starts a downshift. This doesn't match with your description of
> this register, so yes: Implementing the tunable for this PHY doesn't make sense.
True. This register is only going to return 1,2,4 and 8. And it is 
defaulted to 4 attempts.
> However this register may be useful in the link_change_notify() callback to
> figure out whether a downshift happened, to trigger the info message you had in
> your first version.

Thats a good idea but.. The register is defaulted to always report 4 
attempts were made. It never reports 0 attempts so we would never know 
the truth behind the reporting. Kinda like matching the speeds.

Dan