macb: MID register on SAMA5D2 series?

[Alexander Dahl <ada@thorsis.com>]

Hei hei,

while bringing up support for a new SAMA5D27 based board I noticed something 
strange in the macb driver in both U-Boot and Linux. There's a function in 
both to determine if or not the IP block in the SoC is the gigabit variant, 
commonly refered to as GEM.

The function in U-Boot:

	static int macb_is_gem(struct macb_device *macb)
	{
		return MACB_BFEXT(IDNUM, macb_readl(macb, MID)) >= 0x2;
	}

And in Linux:

	static bool hw_is_gem(void __iomem *addr, bool native_io)
	{
	        u32 id;
	
	        if (native_io)
	                id = __raw_readl(addr + MACB_MID);
	        else
	                id = readl_relaxed(addr + MACB_MID);
	
	        return MACB_BFEXT(IDNUM, id) >= 0x2;
	}

In both cases a register MID is read, in both cases that has an offset of 
0x00fc.

	#define MACB_MID				0x00fc

I studied the register layouts in the datasheets for AT91SAM9G20, SAMA5D2 
series, SAMA5D3 series, and SAMA5D4 series. In all but SAMA5D2, offset 0x00fc 
is marked as reserved for both EMAC and GMAC variants.

SAMA5D2 however has a register GMAC_EFTSH (GMAC PTP Event Frame Transmitted 
Seconds High Register) at this offset. Because the check for SAMA5D2 is broken 
in U-Boot since v2017.09-111-g245cbc583d (I will send a patch for that today), 
I got some weird behaviour with our new SAMA5D27 based board. While the 
SAMA5D27-SOM1-EK worked fine in U-Boot, our board did not, but reported 
Gigabit Speed on the ethernet link, which is neither supported by SAMA5D2 nor 
by the ethernet PHY (LAN8720A).

I suppose the register content at that offset on that SoC, just does not give 
that MID? That would be in line with the SAMA5D2 datasheet, and the detection 
on those SoCs currently works or does not only by chance in U-Boot? However 
that register offset was introduced in both U-Boot and Linux long time ago, 
back in 2011 or 2012, so maybe that IP block looks somewhat different on non 
Atmel/Microchip SoCs?

Is there some secret meaning to that register offset, not documented in all 
those Atmel/Microchip datasheets? Or is that check just wrong on those 
platforms and nobody noticed yet?

I would care to send patches, but I would like to get an idea first on what is 
supposed to be in that register. At least I'd like to get the behaviour for 
SAMA5D27 fixed, and would be happy for advice on that. If someone else wants 
to step in, I would happily test it. ;-)

Sorry if I put anyone on Cc, who is not involved in that macb drivers anymore. 
The get-maintainer scripts on both Linux and U-Boot don't return a maintainer, 
so I got some people from the last commits on each. O:-)

Curious greetings
Alex