Hi, On Wed, Dec 2, 2015 at 9:54 PM, Brian Norris <computersforpe...@gmail.com> wrote: > Hi, > > On Wed, Dec 02, 2015 at 09:44:04PM +0100, Jonas Gorski wrote: >> On Wed, Dec 2, 2015 at 9:17 PM, Simon Arlott <si...@fire.lp0.eu> wrote: >> > On 01/12/15 10:41, Jonas Gorski wrote: >> >> On Sat, Nov 28, 2015 at 8:23 PM, Simon Arlott <si...@fire.lp0.eu> wrote: >> >>> + >> >>> + /* Go to start of buffer */ >> >>> + buf -= FC_WORDS; >> >>> + >> >>> + /* Erased if all data bytes are 0xFF */ >> >>> + buf_erased = memchr_inv(buf, 0xFF, FC_WORDS) == NULL; >> >>> + >> >>> + if (!buf_erased) >> >>> + goto out_free; >> >> >> >> We now have a function exactly for that use case in 4.4, >> >> nand_check_erased_buf [1], consider using that. This also has the >> >> benefit of treating bit flips as correctable as long as the ECC scheme >> >> is strong enough. >> > >> > I have no idea whether or not it's appropriate to specify >> > bitflips_threshold > 0 so it'd just be a more complex way to do >> > a memchr_inv() search for 0xFF. >> >> The threshold would be the amount of bitflips the code can correct, so >> basically ecc.strength (at least that is my understanding). >> >> > The code also has to check for the hamming code bytes being all 0x00, >> > because according to the comments [2], the controller also has >> > difficulty with the non-erased all-0xFFs scenario too. >> >> According to brcmnand.c hamming can fix up to fifteen bitflips, but in > > Hamming only protects 1 bitflip. The '15' is the value used by the > controller to represent Hamming (i.e., there is no BCH-15).
Ah, yeah that confused me because I also vaguely remembered hamming only providing protection for 1, but then saw the ecc_level = 15 assignment. Still, that means that even hamming protected erased pages with a single bitflip should be treated as readable / all-0xff, but with correctable bitflips, and not as uncorrectable. >> the current code you would fail a hamming protected all-0xff-page for >> even a single bitflip in the data or in the ecc bytes, which means >> that all-0xff-pages wouldn't be protected at all. > > BTW, I think Kamal had code to handle protecting bitflips in erased > pages code in the Broadcom STB Linux BSP. Perhaps he can port that to > upstream with nand_check_erased_ecc_chunk()? IIUC, that would probably > handle your case too, Simon, although it wouldn't be optimal for an > all-0xff check (i.e., bitflip_threshold == 0). > > If that's really an issue (i.e., we have an implementation + data), I'm > sure we could add optimization to nand_check_erased_ecc_chunk() to > support the bitflip_threshold == 0 case. Maybe I'm missing something, but wasn't the point of introducing nand_check_erased_ecc_chunk that bitflips in erased pages should be treated as bitflips corrected by the ecc, and therefore fixed up before passing the data further on? So having a theshold of 0 would be wrong / no protection at all, and could be quite destructive on MLC nand, where bitflips in erased pages are rather common. Jonas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/