Change in coreboot[master]: src/cpu/power9: add file structure for power9, implement SCOM access

[Krystian Hebel (Code Review)]

Attention is currently required from:  Felix Singer, Arthur Heymans, Sergii Dmytruk, Timothy Pearson, Ron Minnich.
Krystian Hebel has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/57078 )

Change subject: src/cpu/power9: add file structure for power9, implement SCOM access
......................................................................

Patch Set 11:

(1 comment)

File src/include/cpu/power/scom.h:

https://review.coreboot.org/c/coreboot/+/57078/comment/4799aeb8_e78f23f1 
PS11, Line 91: static uint64_t read_scom_direct(uint64_t reg_address)
             : {
             : 	uint64_t val;
             : 	uint64_t hmer = 0;
             : 	do {
             : 		/*
             : 		 * Clearing HMER on every SCOM access seems to slow down CCS up
             : 		 * to a point where it starts hitting timeout on "less ideal"
             : 		 * DIMMs for write centering. Clear it only if this do...while
             : 		 * executes more than once.
             : 		 */
             : 		if ((hmer & SPR_HMER_XSCOM_STATUS) == SPR_HMER_XSCOM_OCCUPIED)
             : 			clear_hmer();
             : 
             : 		eieio();
             : 		asm volatile(
             : 			"ldcix %0, %1, %2" :
             : 			"=r"(val) :
             : 			"b"(MMIO_GROUP0_CHIP0_SCOM_BASE_ADDR),
             : 			"r"(reg_address << 3));
             : 		eieio();
             : 		hmer = read_hmer();
             : 	} while ((hmer & SPR_HMER_XSCOM_STATUS) == SPR_HMER_XSCOM_OCCUPIED);
             : 
             : 	if (hmer & SPR_HMER_XSCOM_STATUS) {
             : 		reset_scom_engine();
             : 		/*
             : 		 * All F's are returned in case of error, but code polls for a set bit
             : 		 * after changes that can make such error appear (e.g. clock settings).
             : 		 * Return 0 so caller won't have to test for all F's in that case.
             : 		 */
             : 		return 0;
             : 	}
             : 	return val;
             : }
             : 
             : static void write_scom_direct(uint64_t reg_address, uint64_t data)
             : {
             : 	uint64_t hmer = 0;
             : 	do {
             : 		/* See comment in read_scom_direct() */
             : 		if ((hmer & SPR_HMER_XSCOM_STATUS) == SPR_HMER_XSCOM_OCCUPIED)
             : 			clear_hmer();
             : 
             : 		eieio();
             : 		asm volatile(
             : 			"stdcix %0, %1, %2"::
             : 			"r"(data),
             : 			"b"(MMIO_GROUP0_CHIP0_SCOM_BASE_ADDR),
             : 			"r"(reg_address << 3));
             : 		eieio();
             : 		hmer = read_hmer();
             : 	} while ((hmer & SPR_HMER_XSCOM_STATUS) == SPR_HMER_XSCOM_OCCUPIED);
             : 
             : 	if (hmer & SPR_HMER_XSCOM_STATUS)
             : 		reset_scom_engine();
             : }
> What is the rationale to not put this in a compilation unit?

None at this moment I guess. It started as just inlined `asm volatile(...); eieio();` but got way more complicated since. This should be moved to scom.c.