[Bug 235944] jedec_dimm(4) does not attach to KFA2 (aka Galax) Hall of Fame DDR4 sticks

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235944

--- Comment #13 from Ravi Pokala <rpokala at panasas.com> ---
(In reply to Andriy Gapon from comment #10)

================================================================
Ravi, are you sure that this is correct?
>                 rc = smbus_writeb(sc->smbus,
>                    (JEDEC_DTI_PAGE | JEDEC_LSA_PAGE_SET1), 0, 0);

Given smbus_writeb(bus,slave,cmd,byte).
================================================================

Sadly, yes, I'm sure it's correct. SMBus only supports 8-bit command codes
(which are used as EEPROM byte offsets), so page-change mechanism is required
for accessing bytes [256-512]. For whatever reason, JEDEC (or the SPD EEPROM
chip vendors) decided that the way to handle that is by writing to a specific
SMBus address. In response, *all* devices on the bus switch to that page. As I
noted in jedec_dimm.h:

================================================================
/* TSE2004av defines several Device Type Identifiers (DTIs), which are the high
 * nybble of the SMBus address. Addresses with DTIs of PROTECT (or PAGE, which
 * has the same value) are essentially "broadcast" addresses; all SPD devices
 * respond to them, changing their mode based on the Logical Serial Address
 * (LSA) encoded in bits [3:1]. For normal SPD access, bits [3:1] encode the
 * DIMM slot number.
 */
================================================================

Thus, using "(JEDEC_DTI_PAGE | JEDEC_LSA_PAGE_SET1)" as the slave address is in
fact correct.

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