[PATCH] eeprom_at24c: Model 8-bit data addressing for 16-bit devices

[Andrew Jeffery <andrew@codeconstruct.com.au>]

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 <andrew@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