On 11/3/2023 4:02 PM, Robert Middleton wrote:
Hi,
I am working on a project that will need to be able to read/write the
ID page of an EEPROM(currently a M95M02 SPI device from ST). From the
code that I have seen, it seems that this chip is already supported,
but there is no code to read/write the device ID page. I'm looking to
add support, but I have some questions:
1. There are currently no IOCTLs defined for EEPROM devices, should I
add a new IOCTL base or add an extra one to the MTD IOCTLs?
See below
2. The default IOCTL for the EEPROM returns -ENOTTY which seems weird,
should that be something else?
ENOTTY is correct. See https://en.wikipedia.org/wiki/Not_a_typewriter
or
https://cwiki.apache.org/confluence/display/NUTTX/ENOTTY+ioctl%28%29+Return+Value
This confuses a lot of people and there are numerous stack overflow
questions like yours.
3. Since you could read/write the ID page, should this really be
implemented as an IOCTL? I would assume that the functions would need
to be more like the read/write syscalls where you pass in a buffer and
the length(assuming you don't want to read or write the entire page).
Would it make sense to make another node in /dev to access this page?
This is related logic for the AT24 EEPROM. That part has an extended
memory region that holds configuration data like that factory
initialized MAC address of the SAMv71-Xult board.
See/boards/arm/samv7/samv71-xult/src/sam_ethernet.c:
/* Configure the AT24 to access the extended memory region */
ret = at24->ioctl(at24, MTDIOC_EXTENDED, 1);
That is not exactly the same functionality, but is, I think sufficiently
related to justify using IOCTLs.
NOTE: That only uses the IOCTL to switch modes of EEPROM. Normal reads
and writes are then used to access the extended range:
/* Read the MAC address */
nread = at24->read(at24, AT24XX_MACADDR_OFFSET, 6, mac);
Using IOCTLs for reads and writes would be awkward.
Optionally, you could treat the control memory as a separate partition,
registering the ID page like it were a different device.
Notice that the AT24 does use an MTD IOCTL. I don't know if we want to
proliferate that naming or not? It would good to re-use it if the
semantics of the name would permit you to re-use it.
