Linux kernel driver: what model for NVRAM access?

Question

I just wrote a RTC driver for an NXP RTC chip on my board, it works great. This chip also has some battery backed RAM that I'd like to make available to a user space application. The RTC framework doesn't support this. It's only 512 bytes but I'm tossed between doing a seekable CHAR driver or a full blown BLOCK driver. I've never done a block driver before but it appears to require a bit more information than a simple CHAR.

I could also interface with IOCTLS but that doesn't feel as clean as it could be. What feels like the best way to make these bytes available to userland?

[EDIT] I forgot to mention that that the RTC chip is hanging off an I2C port, it's not mapped into memory, thus not making it a good candidate for mmaping. [/EDIT]

Answer 1

I think a character device driver implementing mmap should be adequate. Linux Device Drives covers that in chapter 15.

Edit:

Well, i2c is a serial bus, so mmap is not an option. I will refer you to Essential Linux Device Drivers book. I believe it has a sample i2c EEPROM char device driver in Chapter 8. Hope this helps.

Answer 2

Block drivers are only for devices that look like disk drives. Are you going to put a filesystem on your 512 bytes? No? Make it a character device.

You could just do it like other drivers have. Check out drivers/char/nvram.c. That creates a char device you can open(), read(), write(), lseek(), and close().