[RFC] net: stmmac: Problem with adding the native GPIOs support

[Alexandre Torgue]

Hi Serge,

Sorry I never used GPIO provided by DWMAC IP. Obviously, I think is to 
late for you to use GPIOs provided by your SoC directly. Unfortunately, 
it seems to be a "perfect" chicken and eggs problem :(.

Do you have possibilty to "play" with gpio setting. I mean change 
configuration of them (at least for reset one) before perform a DMA 
reset: If you have a pull-up on RST line and you could "disconnect" GPO 
inside GMAC then your PHY should remain on during DMA reset phase.

regards
Alex

On 12/14/20 10:25 AM, Serge Semin wrote:
> Hello folks,
> 
> I've got a problem, which has been blowing by head up for more than three
> weeks now, and I'm desperately need your help in that matter. See our
> Baikal-T1 SoC is created with two DW GMAC v3.73a IP-cores. Each core
> has been synthesized with two GPIOs: one as GPI and another as GPO. There
> are multiple Baikal-T1-based devices have been created so far with active
> GMAC interface usage and each of them has been designed like this:
> 
>   +------------------------+
>   | Baikal-T1 +------------+       +------------+
>   |   SoC     | DW GMAC    |       |   Some PHY |
>   |           |      Rx-clk+<------+Rx-clk      |
>   |           |            |       |            |
>   |           |         GPI+<------+#IRQ        |
>   |           |            |       |            |
>   |           |       RGMII+<----->+RGMII       |
>   |           |        MDIO+<----->+MDIO        |
>   |           |            |       |            |
>   |           |         GPO+------>+#RST        |
>   |           |            |       |            |
>   |           |      Tx-clk+------>+Tx-clk      |
>   |           |            |       |            |
>   |           +------------+       +------------+
>   +------------------------+
> 
> Each of such devices has got en external RGMII-PHY attached configured via the
> MDIO bus with Rx-clock supplied by the PHY and Tx-clock consumed by it. The
> main peculiarity of such configuration is that the DW GMAC GPIOs have been used
> to catch the PHY IRQs and to reset the PHY. Seeing the GPIOs support hasn't
> been added to the STMMAC driver it's the very first setup for now, which has
> been using them. Anyway the hardware setup depicted above doesn't seem
> problematic at the first glance, but in fact it is. See, the DW *MAC driver
> (STMMAC ethernet driver) is doing the MAC reset each time it performs the
> device open or resume by means of the call-chain:
> 
>    stmmac_open()---+
>                    +->stmmac_hw_setup()->stmmac_init_dma_engine()->stmmac_reset().
>    stmmac_resume()-+
> 
> Such reset causes the whole interface reset: MAC, DMA and, what is more
> important, GPIOs as being exposed as part of the MAC registers. That
> in our case automatically causes the external PHY reset, what neither
> the STTMAC driver nor the PHY subsystem expect at all.
> 
> Moreover the stmmac_reset() method polls the DMA_BUS_MODE.SFT_RESET flag
> state to be sure the MAC is successfully completed. But since the external
> PHY has got in reset state it doesn't generate the Rx-clk signal. Due to
> that the MAC-DMA won't get out of the reset state so the stmmac_reset()
> method will return timeout error. Of course I could manually restore the
> GPIOs state in the stmmac_reset() before start to poll the SFT_RESET flag,
> which may release the PHY reset. But that seems more like a workaround,
> because the PHY still has been in reset and need to be reinitialized
> anyway. Moreover some PHY may need to have more complicated reset cycle
> with certain delays between RST assertion/de-assertion, so the workaround
> won't work well for them.
> 
> To sum it up my question is what is the right way to resolve the problem
> described above? My first idea was to just move the MAC reset from the
> net-device open()/close() callbacks to the
> stmmac_dvr_probe()/stmmac_dvr_remove() functions and don't reset the whole
> interface on each device open. The problems we may have in that case is
> due to the suspend/resume procedures, which for some reason require the
> MAC reset too. That's why I need your help in this matter. Do you have any
> idea how to gently add the GPIOs support and don't break the STMMAC
> driver?
> 
> One more tiny question regarding the DW *MAC drivers available in kernel.
> Aside of the DW GMAC Baikal-T1 SoC has also got DW xGMAC v2.11a embedded
> with XPCS PHY attached. My question is what driver should we better use to
> handle our xGMAC interface? AFAICS there are three DW *MAC-related drivers
> the kernel currently provides:
> 1) drivers/net/ethernet/stmicro/stmmac
> 2) drivers/net/ethernet/amd/
> 3) drivers/net/ethernet/synopsys/
> xGBE interface is supported by the drivers 1) and 2). In accordance with
> https://www.spinics.net/lists/netdev/msg414148.html all xGMAC related
> parts should have been added to 2), but recently the XGMAC support has
> been added to 1). So I am confused what driver is now supposed to be used
> for new xGMACs?
> 
> Regards,
> -Sergey
>