Hello again, As I was browsing through the Om community documentation, I saw a note somewhere (I think it was somewhere in the wiki, but I have a hard time finding it again) saying that the native NAND flash on the GTA02 should not be used for anything but the bootloader, and that the OS/distro should live on the micro-SD card instead. No explanation was given as to why. I wonder, would anyone here happen to know the rationale for that recommendation? And does the Om community actually follow that recommendation? Where do most GTA02 users keep their OS/distro: NAND or micro-SD?
Unless I'm missing an elephant somewhere, my common sense suggests that JFFS2 on the native NAND ought to be much more convenient: SD is a serial bus, and it hangs off the Glamo on the GTA02, whereas the native NAND is, well, native, it's right there inside the MCU. It also seems to me that the native NAND ought to be an outright win in terms of power management: NAND is powered from the IO_1V8 rail which is fed from a switching converter in the PMIC, whereas SD is powered via an HCLDO at 3.3 V. The IO_1V8 supply has to stay on all the time anyway as it keeps the SDRAM in self-refresh, so whatever tiny idle current the NAND draws is going to be the same whether one uses it to store a file system or not. But for the SD one needs 3.3 instead of 1.8 V, the linear regulator is inherently more wasteful than a switcher, and what happens when the battery voltage falls below 3.3 V? If I've read the PMIC datasheet correctly, one ought to be able to keep running a GTA02 on battery until it goes down to almost 2.8 V: I reason it ought to be doable with NAND, but not with SD. The only truly valid reason I've been able to come up with for using SD instead of NAND is flash wear concerns. As all good embedded system engineers know, NAND flash is finicky: you've got read disturb, program disturb, and a limited number of erase cycles in each block. Of course the actual physics of NAND flash is exactly the same whether you use it directly or if it's hidden from you behind an interface like SD. The actual "magic smoke" in those SD cards, USB sticks etc is the exact same NAND which people dislike using directly; the only difference is that with native NAND you as a Linux developer get to implement the necessary algorithms yourself (JFFS2 et al) in a fully visible, free / open source manner, whereas with SD cards etc someone else has done that work, in the form of a closed source circuit which you have no visibility into. I tend to agree with Linux MTD maintainer dwmw2's preference of doing it all yourself with JFFS2 et al. Of course the underlying problem of flash wear exhaustion remains there: no matter how good your flash management algorithms are, eventually the available number of erase/program cycles will be exhausted for more blocks than the maximum number of bad blocks one can tolerate. What do we do then? This concern is the ONLY area where I can see an advantage with SD: once the micro-SD card's flash wear cycles have been exhausted (forgetting for the moment that you can't actually depend on the card reporting this condition to you), you simply throw that card in the trash and get a fresh one. OTOH, if you exhaust the available erase/ program cycles for the NAND flash inside that oddball Samsung chip on the GTA02, getting a replacement chip and getting it swapped out on the board would be problematic. Hence my question to the community: is the flash wear-out concern I've just outlined the primary reason for the recommendation of using SD instead of NAND to hold the OS/distro, or was that recommendation driven by some other, completely unrelated concerns? Has anyone here actually exhaused the available number of flash erase/program cycles, on any HW platform, ever? (I personally haven't, in my ~10 y of doing embedded Linux work on various systems: I know from the physics and from device datasheets that the number is finite, I just haven't had a personal experience exhausting it.) Just trying to figure out whether I should use NAND or SD to store the OS/distro on my GTA02 (when I get it), and have a rationale for that choice. And if I do end up wearing out the NAND flash in the Samsung ARM chip on my GTA02... Dr. H. Nikolaus Schaller <[email protected]> wrote: > For us, the GTA02 devices have mostly become sources for plastic parts [...] So when you cannibalize a good GTA02 for its plastic parts (for use with a GTA04 board or whatever), what happens to the good GTA02 PCBA? I hope you don't throw them out, do you? Perhaps those boards could be made available to those hapless GTA02 users who have worn out their NAND flash? Developing a new from-scratch distro generally involves a lot more reloading of the flash than a normal user would do... And while we are on that subject, I wonder, how many of these new-in-box GTA02 devices are still left? At what rate is this legacy stock being depleted? (Unless you consider that a business secret, of course... But methinks the community has a right to know.) MS _______________________________________________ Openmoko community mailing list [email protected] http://lists.openmoko.org/mailman/listinfo/community
