Checking something: Endianness of scom addresses

[hanetzer]

skiboot/hw/xscom.c

Line 108 in f901fca

out_be64(xscom_addr(this_cpu()->chip_id, recv_status_reg), 0);

So I know the p9 can run in either big or little endian mode, but this bit of code leads me to believe that
scom addresses are always big endian, regardless of whether the cpu is running in big or little mode.

Just a bit of clarification would be nice, thank you.

[oohal]

SCOM addresses are just offsets into the XSCOM MMIO area. The SCOMs themselves are big endian MMIO registers so we use in_be64() and out_be64() to access them.

[hanetzer]

Oh, so like, recv_status_reg is at 0x603fc00000000 + recv_status_reg on p9 systems?
And I assume that's true for any other scom reg? Neat.

[oohal]

More or less. Historically SCOMs were accessed from outside the chip via a address/data register pair and the "scom address" needs to be translated from a register number to an offset into the XSCOM MMIO area. For P9 all that requires is shifting the address up by three bits, but it was a little more complex on P8 which is why we use xscom_addr() to do the translation.

The other thing is that each chip has its own xscom MMIO range so 0x603fc00000000 + (scom_addr << 3) is the address for chip 0 and 0x623fc00000000 + (scom_addr << 3) is the same scom on the 2nd chip. xscom_addr() also handles that.