RE: [EXT] Re: [PATCH v2 net 2/2] net: stmmac: dwmac-imx: pause the TXC clock in fixed-link

[Shenwei Wang]

> -----Original Message-----
> From: Will Deacon <will@xxxxxxxxxx>
> Sent: Friday, July 28, 2023 10:36 AM
> To: Russell King (Oracle) <linux@xxxxxxxxxxxxxxx>
> Cc: Andrew Halaney <ahalaney@xxxxxxxxxx>; Shenwei Wang
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> Subject: [EXT] Re: [PATCH v2 net 2/2] net: stmmac: dwmac-imx: pause the TXC
> clock in fixed-link
>
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> On Fri, Jul 28, 2023 at 04:22:19PM +0100, Russell King (Oracle) wrote:
> > On Thu, Jul 27, 2023 at 01:36:45PM -0500, Andrew Halaney wrote:
> > > I don't have any documentation for the registers here, and as you
> > > can see I'm an amateur with respect to memory ordering based on my
> > > prior comment.
> > >
> > > But you:
> > >
> > >     1. Read intf_reg_off into variable iface
> > >     2. Write the RESET_SPEED for the appropriate mode to MAC_CTRL_REG
> > >     3. wmb() to ensure that write goes through
> >
> > I wonder about whether that wmb() is required. If the mapping is
> > device-like rather than memory-like, the write should be committed
> > before the read that regmap_update_bits() does according to the ARM
> > memory model. Maybe a bit of information about where this barrier has
> > come from would be good, and maybe getting it reviewed by the
> > arm64 barrier specialist, Will Deacon. :)
> >
> > wmb() is normally required to be paired with a rmb(), but we're not
> > talking about system memory here, so I also wonder whether wmb() is
> > the correct barrier to use.
>
> Yes, I don't think wmb() is the right thing here. If you need to ensure that the
> write to MAC_CTRL_REG has taken effect, then you'll need to go through some
> device-specific sequence which probably involves reading something back. If you
> just need things to arrive in order eventually, the memory type already gives you
> that.
>
> It's also worth pointing out that udelay() isn't necessarily ordered wrt MMIO
> writes, so that usleep_range() might need some help as well.
> Non-relaxed MMIO reads, however, _are_ ordered against a subsequent
> udelay(), so if you add the readback then this might all work out.
>

    1. Write RESET_SPEED
    2. Write 0 to MX93_GPR_ENET_QOS_INTF_MODE_MASK
    3. usleep_range()
    4. Restore MX93_GPR_ENET_QOS_CLK_GEN_EN | iface

In the above example, if a readback after step #2 could ensure sufficient time has passed
before step #4, the wmb() here should be abandoned.

Thanks,
Shenwei

> I gave a (slightly dated) talk about some of this at ELC a while back:
>
> https://www.yo/
> utube.com%2Fwatch%3Fv%3Di6DayghhA8Q&data=05%7C01%7Cshenwei.wang
> %40nxp.com%7C32396fd0396e4e46975f08db8f806680%7C686ea1d3bc2b4c6fa
> 92cd99c5c301635%7C0%7C0%7C638261553857503588%7CUnknown%7CTWFp
> bGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6
> Mn0%3D%7C3000%7C%7C%7C&sdata=X5CQrQEVmUjYafYJ%2BzcnGXI9mhDT%
> 2BMzDazGHOcoomas%3D&reserved=0
>
> which might help.
>
> Will