[linux-sunxi] Re: [PATCH 04/10] net: stmmac: sunxi platfrom extensions for GMAC in Allwinner A20 SoC's

[Chen-Yu Tsai]

Hi,

On Tue, Dec 10, 2013 at 12:16 AM, Hans de Goede <hdegoede at redhat.com> wrote:
> Hi,
>
>
> On 12/09/2013 12:10 PM, srinivas kandagatla wrote:
>>
>> Hi Chen,
>> Good to know that Allwinner uses gmac.
>>
>> On ST SoC, we have very similar requirements, before we merge any of
>> these changes I think we need to come up with common way to solve both
>> Allwinner and ST SOCs use cases.
>>
>> I have already posted few patches on to net-dev
>> http://lkml.org/lkml/2013/11/12/243 to add Glue driver on top of stmmac
>> driver.
>>
>>
>> There are few reasons for the way I have done it.
>>
>> 1> I did not want to modify gmac driver in any sense to when a new SOC
>> support is added.
>> 2> As the SOC specific glue logic sits on top of standard GMAC IP, it
>> makes sense to represent it proper harware hierarchy.
>
>
> On top often is not the correct term / how things are done in practice.
>
> Most of the time the glue are modifications to the ip block, iow in
> hardware they are not nested / hierarchical at all. We've had similar
> constructs for ahci platform drivers and there we are actively trying to
> move away from the whole nested platform devices as that has various
> issues:
>
> 1) It is wrong / does not reflect reality
> 2) It breaks deferred probing which is often used on SOCs
>
> I actually think Wens' approach using a SOC specific init function
> in platform data is sound, and this is also used a lot else where.
>
> As for using the nested approach elsewhere, I only know about AHCI
> platform driver doing that, and there we are actively trying to move
> away from it.
>
>
> Now reading this has also made me take a closer look at wens' patch
> for this. Wens, I see that you directly modify registers in the ccm
> that is a big no-no instead you should add a helper function to
> sunxi-clk.c and use that, see ie:
> https://bitbucket.org/emiliolopez/linux/commits/2b95847d9aa4aa13317dd7358ffcbd951dcb5eff?at=master

Yes, this has been raised by Maxime. The odd "GMAC_IF_TYPE_RGMII" or
"gmac interface type bit" has been bugging me.

Additionally, the TX clock has 2 inputs (not counting MII [1]).
The internal one is most likely controlled by the GMAC. The clock
rate is set internally to match the link speed. The external clock
source has controllable dividers to get the correct clock rate.
This shouldn't be hard to model with CCF though.

In hardware, this is probably a mux between the GMAC clock generator
and the GMAC data transmit logic.

My current plan is to choose MII when the clock is disabled,
and choose either of the inputs when it is enabled. I will
have to learn more about the CCF first.

[1] In MII mode, the TX clock is sent by the PHY to the MAC.
    From the MAC's viewpoint, it seems like a clock source.
    However, from the PHY's viewpoint, it is the provider.
    In RGMII mode, the MAC provides the TX clock to the PHY,
    and times the TX data lines to the clock. So in a way,
    the PHY is the true consumer of the TX clock.


ChenYu

> Giuseppe, I get the feeling the 0x148 registers st-gmac is using is
> the same story, but I could be wrong. One important reason why accessing
> regs like this directly is a big no no is because it is impossible to
> do proper locking between multiple users, so 2 users doing read / modify /
> write
> could end up racing. So accesses to such registers should always happen
> through a single driver.
>
>
> Regards,
>
> Hans