The Linux-Embedded discussion list has been abuzz the last two days over a flash filesystem designed to support binary code execution (sometimes called XIP, or “execute-in-place”). When combined with forthcoming “Phase Change Memory” products, the “Advanced XiP Filesystem” (AXFS) could radically change the way Linux is embedded on consumer devices.

The AXFS project is led by Jared Hulbert, who works for Numonyx. A joint venture of Intel, STMicroelectronics, and Francisco Partners, Numonyx was founded in March of this year to commercialize “phase change memory (PCM),” among other opportunities in the non-volatile memory market.

Using heat rather than electricity to record data, PCM boasts bit-level rather than page-level alterability, multi-level cells, and, says Numonyx, overall performance characteristics that have some calling it a DRAM replacement, as well as a NAND replacement. In a recent talk at the Ottawa Linux Symposium, Hulbert said, “PCM is going to make XiP a lot more relevant.”

The AXFS project has been under development for quite a while, and was even previously submitted to the LKML (Linux kernel mailing list) for possible merger into mainline Linux. In a nutshell, AXFS is a read-only filesystem resembling Cramfs — which can also be used for “linear XiP.” A key difference is that Cramfs decompresses at the file level, whereas AXFS decompresses at the page level.

In preparation to submit AXFS code to mainline a second time, Hulbert posted 11 patches totalling 2,872 lines of code to the new Linux-Embedded list, asking the hardcore embedded developers there to help with a “first round of review.” In about 60 replies, developers weighed in with suggestions, encouragement, thanks, praise, questions, and comments.

Suggestions ranged from code cleanup options to feature additions, such as “fake-writes.” The choice of names was also discussed, along with possible ways to use the filesystem on systems without memory management hardware, and possible applications of AXFS apart from XIP, such as in RAM disks and on S390 servers running from NAND flash drives.

More can be learned from the Linux Embedded archives, Hulbert's recent OLS presentation (ogg video), or from a vendor-neutral whitepaper on XIP alternatives co-authoed by Hulbert.

 
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