It appears some (many?) EEPROMs that implement 16-bit data addressing will accept an 8-bit address and clock out non-uniform data for the read. This behaviour is exploited by an EEPROM detection routine in part of OpenBMC userspace with a reasonably broad user base:
https://github.com/openbmc/entity-manager/blob/0422a24bb6033605ce75479f675fedc76abb1167/src/fru_device.cpp#L197-L229 The diversity of the set of EEPROMs that it operates against is unclear, but this code has been around for a while now. Separately, The NVM Express Management Interface Specification dictates the provided behaviour in section 8.2 Vital Product Data: > If only one byte of the Command Offset is provided by the Management > Controller, then the least significant byte of the internal offset > shall be set to that value and the most-significant byte of the > internal offset shall be cleared to 0h https://nvmexpress.org/wp-content/uploads/NVM-Express-Management-Interface-Specification-1.2c-2022.10.06-Ratified.pdf This change makes it possible to expose NVMe VPD in a manner that can be dynamically detected by OpenBMC. Signed-off-by: Andrew Jeffery <and...@codeconstruct.com.au> --- hw/nvram/eeprom_at24c.c | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/hw/nvram/eeprom_at24c.c b/hw/nvram/eeprom_at24c.c index 613c4929e327..64a61cc0e468 100644 --- a/hw/nvram/eeprom_at24c.c +++ b/hw/nvram/eeprom_at24c.c @@ -98,12 +98,20 @@ uint8_t at24c_eeprom_recv(I2CSlave *s) EEPROMState *ee = AT24C_EE(s); uint8_t ret; - /* - * If got the byte address but not completely with address size - * will return the invalid value - */ if (ee->haveaddr > 0 && ee->haveaddr < ee->asize) { - return 0xff; + /* + * Provide behaviour that aligns with NVMe MI 1.2c, section 8.2. + * + * https://nvmexpress.org/wp-content/uploads/NVM-Express-Management-Interface-Specification-1.2c-2022.10.06-Ratified.pdf + * + * Otherwise, the clocked-out data is meaningless anyway, and so reading + * off memory is as good a behaviour as anything. This also happens to + * help the address-width detection heuristic in OpenBMC's userspace. + * + * https://github.com/openbmc/entity-manager/blob/0422a24bb6033605ce75479f675fedc76abb1167/src/fru_device.cpp#L197-L229 + */ + ee->haveaddr = ee->asize; + ee->cur %= ee->rsize; } ret = ee->mem[ee->cur]; -- 2.39.2
