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Date:   Mon, 25 May 2020 23:01:27 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, andrew@...n.ch,
        f.fainelli@...il.com, hkallweit1@...il.com,
        madalin.bucur@....nxp.com, calvin.johnson@....nxp.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 04/11] net: phy: Handle c22 regs presence better

On Mon, May 25, 2020 at 04:51:16PM -0500, Jeremy Linton wrote:
> Hi,
> 
> On 5/25/20 5:06 AM, Russell King - ARM Linux admin wrote:
> > On Sun, May 24, 2020 at 10:34:13PM -0500, Jeremy Linton wrote:
> > > Hi,
> > > 
> > > On 5/23/20 1:37 PM, Russell King - ARM Linux admin wrote:
> > > > On Fri, May 22, 2020 at 04:30:52PM -0500, Jeremy Linton wrote:
> > > > > Until this point, we have been sanitizing the c22
> > > > > regs presence bit out of all the MMD device lists.
> > > > > This is incorrect as it causes the 0xFFFFFFFF checks
> > > > > to incorrectly fail. Further, it turns out that we
> > > > > want to utilize this flag to make a determination that
> > > > > there is actually a phy at this location and we should
> > > > > be accessing it using c22.
> > > > > 
> > > > > Signed-off-by: Jeremy Linton <jeremy.linton@....com>
> > > > > ---
> > > > >    drivers/net/phy/phy_device.c | 16 +++++++++++++---
> > > > >    1 file changed, 13 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
> > > > > index f0761fa5e40b..2d677490ecab 100644
> > > > > --- a/drivers/net/phy/phy_device.c
> > > > > +++ b/drivers/net/phy/phy_device.c
> > > > > @@ -689,9 +689,6 @@ static int get_phy_c45_devs_in_pkg(struct mii_bus *bus, int addr, int dev_addr,
> > > > >    		return -EIO;
> > > > >    	*devices_in_package |= phy_reg;
> > > > > -	/* Bit 0 doesn't represent a device, it indicates c22 regs presence */
> > > > > -	*devices_in_package &= ~BIT(0);
> > > > > -
> > > > >    	return 0;
> > > > >    }
> > > > > @@ -742,6 +739,8 @@ static int get_phy_c45_ids(struct mii_bus *bus, int addr, u32 *phy_id,
> > > > >    	int i;
> > > > >    	const int num_ids = ARRAY_SIZE(c45_ids->device_ids);
> > > > >    	u32 *devs = &c45_ids->devices_in_package;
> > > > > +	bool c22_present = false;
> > > > > +	bool valid_id = false;
> > > > >    	/* Find first non-zero Devices In package. Device zero is reserved
> > > > >    	 * for 802.3 c45 complied PHYs, so don't probe it at first.
> > > > > @@ -770,6 +769,10 @@ static int get_phy_c45_ids(struct mii_bus *bus, int addr, u32 *phy_id,
> > > > >    		return 0;
> > > > >    	}
> > > > > +	/* Bit 0 doesn't represent a device, it indicates c22 regs presence */
> > > > > +	c22_present = *devs & BIT(0);
> > > > > +	*devs &= ~BIT(0);
> > > > > +
> > > > >    	/* Now probe Device Identifiers for each device present. */
> > > > >    	for (i = 1; i < num_ids; i++) {
> > > > >    		if (!(c45_ids->devices_in_package & (1 << i)))
> > > > > @@ -778,6 +781,13 @@ static int get_phy_c45_ids(struct mii_bus *bus, int addr, u32 *phy_id,
> > > > >    		ret = _get_phy_id(bus, addr, i, &c45_ids->device_ids[i], true);
> > > > >    		if (ret < 0)
> > > > >    			return ret;
> > > > > +		if (valid_phy_id(c45_ids->device_ids[i]))
> > > > > +			valid_id = true;
> > > > 
> > > > Here you are using your "devices in package" validator to validate the
> > > > PHY ID value.  One of the things it does is mask this value with
> > > > 0x1fffffff.  That means you lose some of the vendor OUI.  To me, this
> > > > looks completely wrong.
> > > 
> > > I think in this case I was just using it like the comment in
> > > get_phy_device() "if the phy_id is mostly F's, there is no device here".
> > > 
> > > My understanding is that the code is trying to avoid the 0xFFFFFFFF returns
> > > that seem to indicate "bus ok, phy didn't respond".
> > > 
> > > I just checked the OUI registration, and while there are a couple OUI's
> > > registered that have a number of FFF's in them, none of those cases seems to
> > > overlap sufficiently to cause this to throw them out. Plus a phy would also
> > > have to have model+revision set to 'F's. So while might be possible, if
> > > unlikely, at the moment I think the OUI registration keeps this from being a
> > > problem. Particularly, if i'm reading the mapping correctly, the OUI mapping
> > > guarantees that the field cannot be all '1's due to the OUI having X & M
> > > bits cleared. It sort of looks like the mapping is trying to lose those
> > > bits, by tossing bit 1 & 2, but the X & M are in the wrong octet (AFAIK, I
> > > just read it three times cause it didn't make any sense).
> > 
> > I should also note that we have at least one supported PHY where one
> > of the MMDs returns 0xfffe for even numbered registers and 0x0000 for
> > odd numbered registers in one of the vendor MMDs for addresses 0
> > through 0xefff - which has a bit set in the devices-in-package.
> > 
> > It also returns 0x0082 for almost every register in MMD 2, but MMD 2's
> > devices-in-package bit is clear in most of the valid MMDs, so we
> > shouldn't touch it.
> > 
> > These reveal the problem of randomly probing MMDs - they can return
> > unexpected values and not be as well behaved as we would like them to
> > be.  Using register 8 to detect presence may be beneficial, but that
> > may also introduce problems as we haven't used that before (and we
> > don't know whether any PHY that wrong.)  I know at least the 88x3310
> > gets it right for all except the vendor MMDs, where the low addresses
> > appear non-confromant to the 802.3 specs.  Both vendor MMDs are
> > definitely implemented, just not with anything conforming to 802.3.
> 
> Yes, we know even for the NXP reference hardware, one of the phy's doesn't
> probe out correctly because it doesn't respond to the ieee defined
> registers. I think at this point, there really isn't anything we can do
> about that unless we involve the (ACPI) firmware in currently nonstandard
> behaviors.
> 
> So, my goals here have been to first, not break anything, and then do a
> slightly better job finding phy's that are (mostly?) responding correctly to
> the 802.3 spec. So we can say "if you hardware is ACPI conformant, and you
> have IEEE conformant phy's you should be ok". So, for your example phy, I
> guess the immediate answer is "use DT" or "find a conformant phy", or even
> "abstract it in the firmware and use a mailbox interface".

You haven't understood.  The PHY does conform for most of the MMDs,
but there are a number that do not conform.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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