[PATCH 3/4] [RFC] ARM: dts: stm32: Add mux for ETHRX clock

[Alexandre TORGUE]

On 1/15/21 1:15 PM, Marek Vasut wrote:
> On 1/14/21 6:08 PM, Alexandre TORGUE wrote:
>> Hi Marek
> 
> Hi,
> 
>> On 1/6/21 9:43 PM, Marek Vasut wrote:
>>> The implementation of ETH_RX_CLK/ETH_REF_CLK handling currently does not
>>> permit selecting the clock input from SoC pad. To make things worse, the
>>> implementation of this is partly present and is split between the clock
>>> driver and dwmac4 driver. Moreover, the ETHRX clock parent is incorrect.
>>
>> Sorry but I don't understand which configuration is missing. I think 
>> we can handle all possible cases for RMII. At the glue layer 
>> (dwmac-stm32.c) clocks gates and syscfg are set regarding device tree 
>> binding (see the tab in dwmac-stm32.c). You could have a look here for 
>> more details: 
>> https://wiki.st.com/stm32mpu/wiki/Ethernet_device_tree_configuration
>>
>> Regarding the clock parent, yes it is not at the well frequency if you 
>> want to select this path. Our current "clock tree" is done to fit with 
>> our ST reference boards (we have more peripherals than PLL outputs so 
>> we have to make choices). So yes for customer/partners boards this 
>> clock tree has to be modified to better fit with the need (either 
>> using assigned-clock-parent or by modifying bootloader clock tree 
>> (tf-a or u-boot)).
> 
> I don't think you handle all the configuration options, but I might also 
> be confused.
> 
> See Figure 83. Peripheral clock distribution for Ethernet in the MP1 
> datasheet for the below.
> 
> The current setup I have needs 50 MHz on SoC pad PA1 to drive the PHY 
> clock, and uses eth_clk_fb to supply ETH_RX_CLK. However, the 50 MHz is 
> sourced directly from PLL4P, which then has to run at 50 MHz and that in 
> turn reduces clock frequency for other blocks connected to PLL4P (e.g. 
> SDMMC, where the impact is noticable).

Ok that's the common path to clock a PHY a 50MHz without using the 
ref_clk coming from the PHY. And yes I can understand that the drawback 
is huge).

> 
> So, what I want to model here is this:
> 
> PLL4P = 100 MHz
> MCO2 is supplied by PLL4P and set to /2 , so MCO2 = 50 MHz
> SoC pad PG2 is set as MCO2 output, thus a source of 50 MHz signal
> SoC pad PA1 is set as ETH_RX_CLK and connected to PG2

Ok I see (to be honest IIWR we didn't test i :$) but it should work.

> This works fine in practice, except it cannot be modeled using current 
> DT bindings, even though it should be possible to model it.

For dwmac point of view it's quite the same thing to have your PHY 
clocking by MCO or by a crystal. You just need to configure RX_REF pad 
and ETH_CLK_SEL to get the 50 MHz RMII reference clock.

> 
>>> First, the ETHRX clock in clk-stm32mp1.c only represents the ETHRXEN 
>>> gate,
>>> however it should represent also ETH_REF_CLK_SEL mux. The problem is 
>>> that
>>> the ETH_REF_CLK_SEL mux is currently configured in the DWMAC4 driver and
>>> the ETH_REF_CLK_SEL bit is part of SYSCFG block, not the DWMAC4 or the
>>> clock block.
>>
>> dwmac4-stm32 doesn't contain code for dwmac4 but it contains the glue 
>> around the dwmac4: syscfg, clocks ...
> 
> The problem is that dwmac4-stm32 isn't the right place to configure the 
> ETHRX clock mux, that should be in the clock driver. So the stm32 clock 
> driver should have SYSCFG handle and configure ETH_REF_CLK_SEL mux. The 
> "st,eth-ref-clk-sel" DT prop would then not be needed at all, as the 
> reference clock select would be configured using assigned-clocks in DT.

Idea was to keep at the same place the Ethernet glue configuration. We 
can't move all this glue into clock driver as phy interface is needed to 
well configure some sysconf registers. Current dwamc-stm32 glue is 
working and documented. I'm not convinced to develop a new one by 
splitting clock sysconf in clock driver and phy interface management at 
ethernet level. I think we will get the same functional result (but yes 
maybe more understandable at dt-bindings level). We could maybe update 
binding name to be more clear.

> 
> The default assigned-clocks should be eth_clk_fb , but the user can 
> override it in the DT and provide another clock source (e.g. in my case, 
> that would be PLL4P->MCO2->ETHRX).
> 
>>> Second, the ETHRX parent clock is either eth_clk_fb (ETHCK_K) or 
>>> external
>>> ETH_RX_CLK/ETH_REF_CLK_SEL, it is never CK_AXI.
>>
>> Why CK_AXI ?
> 
> See drivers/clk/clk-stm32mp1.c:
>    1895          PCLK(ETHRX, "ethrx", "ck_axi", 0, G_ETHRX),
>

Ok I see, and it is the same case for TX also. Discussing with our clock 
expert it was done for simplification.

regards
Alex


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