Re: [PATCH v5 net-next 5/9] enetc: Make MDIO accessors more generic and export to include/linux/fsl

[Florian Fainelli <f.fainelli@gmail.com>]

On 1/6/2020 3:00 PM, Vladimir Oltean wrote:
> Hi Florian,
> 
> On Mon, 6 Jan 2020 at 21:35, Florian Fainelli <f.fainelli@gmail.com> wrote:
>>
>> On 1/5/20 5:34 PM, Vladimir Oltean wrote:
>>> From: Claudiu Manoil <claudiu.manoil@nxp.com>
>>>
>>> Within the LS1028A SoC, the register map for the ENETC MDIO controller
>>> is instantiated a few times: for the central (external) MDIO controller,
>>> for the internal bus of each standalone ENETC port, and for the internal
>>> bus of the Felix switch.
>>>
>>> Refactoring is needed to support multiple MDIO buses from multiple
>>> drivers. The enetc_hw structure is made an opaque type and a smaller
>>> enetc_mdio_priv is created.
>>>
>>> 'mdio_base' - MDIO registers base address - is being parameterized, to
>>> be able to work with different MDIO register bases.
>>>
>>> The ENETC MDIO bus operations are exported from the fsl-enetc-mdio
>>> kernel object, the same that registers the central MDIO controller (the
>>> dedicated PF). The ENETC main driver has been changed to select it, and
>>> use its exported helpers to further register its private MDIO bus. The
>>> DSA Felix driver will do the same.
>>
>> This series has already been applied so this may be food for thought at
>> this point, but why was not the solution to create a standalone mii_bus
>> driver and have all consumers be pointed it?
>>
> 
> I have no real opinion on this.
> 
> To be honest, the reason is that the existing "culture" of Freescale
> MDIO drivers wasn't to put them in drivers/net/phy/mdio-*.c, and I
> just didn't look past the fence.
> 
> But what is the benefit? What gets passed between bcmgenet and
> mdio-bcm-unimac with struct bcmgenet_platform_data is equivalent with
> what gets passed between vsc9959 and enetc_mdio with the manual
> population of struct mii_bus and struct enetc_mdio_priv, no? I'm not
> even sure there is a net reduction in code size. And I am not really
> sure that I want an of_node for the MDIO bus platform device anyway.
> Whereas genet seems to be instantiating a port-private MDIO bus for
> the _real_ (but nonetheless embedded) PHY, the MDIO bus we have here
> is for the MAC PCS, which is more akin to the custom device tree
> binding "pcsphy-handle" that the DPAA1 driver is using (see
> arch/arm64/boot/dts/qoriq-fman3-0-10g-0.dtsi for example). So there is
> no requirement to run the PHY state machine on it, it's just locally
> driven, so I don't want to add a dependency on device tree where it's
> really not needed. (By the way I am further confused by the
> undocumented/unused "brcm,40nm-ephy" compatible string that these
> device tree bindings for genet have).

That compatibility string should not have been defined, but the DTS were
imported from our Device Tree auto-generation tool that did produce
those, when my TODO list empty, I might send an update to remove those,
unless someone thinks it's ABI and it would break something (which I can
swear won't).

> 
>> It is not uncommon for MDIO controllers to be re-used and integrated
>> within a larger block and when that happens whoever owns the largest
>> address space, say the Ethernet MAC can request the large resource
>> region and the MDIO bus controler can work on that premise, that's what
>> we did with genet/bcmmii.c and mdio-bcm-unimac.c for instance (so we
>> only do an ioremap, not request_mem_region + ioremap).
>>
> 
> I don't really understand this. In arch/mips/boot/dts, for all of
> bcm73xx and bcm74xx SoCs, you have a single Ethernet port DT node, and
> a single MDIO bus as a child beneath it, where is this reuse that you
> mention?
> And because I don't really understand what you've said, my following
> comment maybe makes no sense, but I think what you mean by "MDIO
> controller reuse" is that there are multiple instantiations of the
> register map, but ultimately every transaction ends up on the same
> MDIO/MDC pair of wires and the same electrical bus.
> We do have some of that with the ENETC, but not with the switch, whose
> internal MDIO bus has no connection to the outside world, it just
> holds the PCS front-ends for the SerDes.
> I also don't understand the reference to request_mem_region, perhaps
> it would help if you could show some code.

What I forgot telling you about is that the same MDIO bus controller is
used internally by each GENET instance to "talk" to both external and
internal PHYs, but also by the bcm_sf2.c driver which is why it made
sense to have a standalone MDIO bus driver that could either be
instantiated on its own (as is the case with bcm_sf2) or as part of a
larger block within GENET. The request_mem_region() + ioremap() comment
is because you cannot have two resources that overlap be used with
request_mem_region(), since the MDIO bus driver is embedded into a
larger block, it simply does an ioremap. If that confused you, then you
can just discard that comment, is it not particularly relevant.

> 
>> Your commit message does not provide a justification for why this
>> abstraction (mii_bus) was not suitable or considered here. Do you think
>> that could be changed?
>>
> 
> I'm sorry, was the mii_bus abstraction really not considered here?
> Based on the stuff exported in this patch, an mii_bus is exactly what
> I'm registering in 9/9, no?

I meat in the commit message, there is no justification why this was not
considered or used, by asking you ended up providing one, that is
typically what one would expect to find to explain why something was/was
not considered. It's fine, the code is merge, I won't object or require
you to use a mii_bus abstraction.
-- 
Florian