> > Hi Andy > > > > Thanks for digging into the internal of the FEC. Just to make sure i > > understand > > this correctly: > > > > In fec_enet_mii_init() we have: > > > > holdtime = DIV_ROUND_UP(clk_get_rate(fep->clk_ipg), 100000000) > > - 1; > > > > fep->phy_speed = mii_speed << 1 | holdtime << 8; > > > > writel(fep->phy_speed, fep->hwp + FEC_MII_SPEED); > > > > /* Clear any pending transaction complete indication */ > > writel(FEC_ENET_MII, fep->hwp + FEC_IEVENT); > > > > You are saying this write to the FEC_MII_SPEED register can on some SoCs > > trigger an FEC_ENET_MII event. And because it does not happen immediately, > > it happens after the clear which is performed here? > > Correct. > Before write FEC_MII_SPEED register, FEC_MII_DATA register is not zero, and > the current value of FEC_MII_SPEED register is zero, once write non zero value > to FEC_MII_SPEED register, it trigger MII event. > > > Sometime later we then go into fec_enet_mdio_wait(), the event is still > > pending, so we read the FEC_MII_DATA register too early? > > Correct. > The first mdio operation is mdio read, read FEC_MII_DATA register is too > early, > it get invalid value. > > > > But this does not fully explain the problem. This should only affect the > > first > > MDIO transaction, because as we exit fec_enet_mdio_wait() the event is > > cleared. But Leonard reported that all reads return 0, not just the first. > > Of course, it impact subsequent mdio read/write operations. > After you clear MII event that is pending before. > Then, after mdio read data back, MII event is set again. > > cpu instruction is much faster than mdio read/write operation.
Ah. Now i get it.... The flow is... Write FEC_MII_SPEED register Clear event register A little latter event bit set because of FEC_MII_SPEED A little later Setup read fec_enet_mdio_wait() exit immediately, because of pending event from FEC_MII_SPEED Clear FEC_MII_SPEED event Read data register too early A little later event bit set because read complete A little later Setup read fec_enet_mdio_wait() exit immediately, because of pending event from read complete Clear previous read complete event Read data register too early A little later event bit set because read complete And the cycle continues... I will make a formal patch from your email. Andrew
