Hi Calvin, >From: Artem Bityutskiy [mailto:artem.bityuts...@linux.intel.com] >>On Mon, 2014-01-13 at 17:49 +0530, Calvin Johnson wrote: >> If the 4th to last-page are left blank and not covered with ECC, what >> will happen in case of bit flips on the blank pages? There was an >> issue reported some time back. >> http://lists.infradead.org/pipermail/linux-mtd/2012-January/039256.html >> >> Does UBI/UBIFS take care of this now? > >No. UBIFS still assumes that blank pages are ECC-protected by the >driver. No one stepped in and took care of changing this yet. > Yes, it's true that in newer technologies (specially < 28nm flash), we are seeing lot of erased-pages having bit-flips. And due to which UBIFS is cribbing. But as there is no ECC stored in an erased-page, bit-flips in erased-page cannot be corrected, unless you compare each byte of read_data. However, there are other way of handling bit-flips in erased-page. Following are few ways in which OMAP NAND driver handles bit-flips in erased-page:
*Case-1*: If bit-flips are found in data-region of an erased-page. (1) An erased-page implicitly means that its data-region should *only* contain 0xff, so its safe to fill read_buf() with 0xff. But the controller driver should report the correctable/un-correctable bit-flips to upper-layer, so that upper-layers like UBI take corrective action by re-erase this block before using it. (Refer) http://lists.infradead.org/pipermail/linux-mtd/2014-January/051368.html *Case-2*: If bit-flips are found in oob-region of an erased-page. This is bit trivial, because if there are bit-flips in ecc-layout (OOB region) of erased-page, it would be difficult to differentiate between an erased-page v/s programmed-page. Though you can keep a 'marker' in ecc-layout reserved for detecting Programmed-pages, but that marker byte itself can be subjected to bit-flips (assuming on MLC and newer technology NAND bit-flips are common). So, OMAP NAND driver takes probabilistic approach. (Refer) http://lists.infradead.org/pipermail/linux-mtd/2014-January/051367.html ----------------------- This patch 'assumes' any page to be 'erased': (a) if all(read_ecc) == 0xff (b) else if all(read_data) == 0xff ----------------------- Currently both UBI and UBIFS layer checks for erased-page to be all(0xff), But I think its over-kill to put this burden on UBI or UBIFS layer, because low-level controller drivers can handle this easily. So, if Artem and Brian agree to above approaches, then I can a submit patch for removal of: - "ubi_self_check_all_ff()" from UBI layer. - checking of 'buf == 0xff' from ubifs_scan_leb() in UBIFS layer. with regards, pekon _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
