Re: [PATCH RFC net-next 12/13] net: phylink: add struct phylink_pcs

[Ioana Ciornei <ioana.ciornei@nxp.com>]

On 7/20/20 7:26 PM, Russell King - ARM Linux admin wrote:
> On Mon, Jul 20, 2020 at 03:50:57PM +0000, Ioana Ciornei wrote:
>> On 6/30/20 5:29 PM, Russell King wrote:
>>> Add a way for MAC PCS to have private data while keeping independence
>>> from struct phylink_config, which is used for the MAC itself. We need
>>> this independence as we will have stand-alone code for PCS that is
>>> independent of the MAC.  Introduce struct phylink_pcs, which is
>>> designed to be embedded in a driver private data structure.
>>>
>>> This structure does not include a mdio_device as there are PCS
>>> implementations such as the Marvell DSA and network drivers where this
>>> is not necessary.
>>>
>>> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
>>
>> Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
>>
>> I integrated and used the phylink_pcs structure into the Lynx PCS just
>> to see how everything fits. Pasting below the main parts so that we can
>> catch early any possible different opinions on how to integrate this:
>>
>> The basic Lynx structure looks like below and the main idea is just to
>> encapsulate the phylink_pcs structure and the mdio device (which in some
>> other cases might not be needed).
>>
>> struct lynx_pcs {
>>          struct phylink_pcs pcs;
>>          struct mdio_device *mdio;
>>          phy_interface_t interface;
>> };
>>
>> The lynx_pcs structure is requested by the MAC driver with:
>>
>> struct lynx_pcs *lynx_pcs_create(struct mdio_device *mdio)
>> {
>> (...)
>>          lynx_pcs->mdio = mdio;
>>          lynx_pcs->pcs.ops = &lynx_pcs_phylink_ops;
>>          lynx_pcs->pcs.poll = true;
>>
>>          return lynx_pcs;
>> }
>>
>> And then passed to phylink with something like:
>>
>> phylink_set_pcs(pl, &lynx_pcs->pcs);
>>
>>
>> For DSA it's a bit less straightforward because the .setup() callback
>> from the dsa_switch_ops is run before any phylink structure has been
>> created internally. For this, a new DSA helper can be created that just
>> stores the phylink_pcs structure per port:
>>
>> void dsa_port_phylink_set_pcs(struct dsa_switch *ds, int port,
>>                                struct phylink_pcs *pcs)
>> {
>>          struct dsa_port *dp = dsa_to_port(ds, port);
>>
>>          dp->pcs = pcs;                                         but I do
>> }
>>
>> and at the appropriate time, from dsa_slave_setup, it can really install
>> the phylink_pcs with phylink_set_pcs.
>> The other option would be to add a new dsa_switch ops that requests the
>> phylink_pcs for a specific port - something like phylink_get_pcs.
> 
> It is entirely possible to set the PCS in the mac_prepare() or
> mac_config() callbacks - but DSA doesn't yet support the mac_prepare()
> callback (because it needs to be propagated through the DSA way of
> doing things.)
> 
> An example of this can be found at:
> 
> http://git.armlinux.org.uk/cgit/linux-arm.git/commit/?h=mcbin&id=593d56ef8c7f7d395626752f6810210471a5f5c1
> 
> where we choose between the XLGMAC and GMAC pcs_ops structures
> depending on the interface mode we are configuring for.  Note that
> this is one of the devices that the PCS does not appear as a
> distinctly separate entity in the register set, at least in the
> GMAC side of things.
> 

Thanks for the info, I didn't get that this is possible by reading the 
previous patch. Maybe this would be useful in the documentation of the 
callback?

Back to the DSA, I don't feel like we gain much by setting up the 
phylink_pcs from another callback: we somehow force the driver to 
implement a phylink_mac_prepare dsa_switch_ops just so that it sets up 
the phylink_pcs, which for me at least would not be intuitive.

Ioana