Hi Boris, 2018-08-24 21:55 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxx>: > Hi Masahiro, > > On Tue, 21 Aug 2018 17:23:19 +0900 > Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote: > >> Commit 49aa76b16676 ("mtd: rawnand: do not execute nand_scan_ident() >> if maxchips is zero") gave a new meaning for calling nand_scan_ident() >> with maxchips=0. >> >> It is a special usage for some drivers such as docg4, but in fact >> the Denali driver may pass maxchips=0 to nand_scan() when the driver >> is enabled but no NAND chip is found on the board for some reasons. >> >> If nand_scan_with_ids() is called with maxchips=0, nand_scan_ident() >> is skipped, i.e. nand_set_defaults() is skipped. Therefore, the >> driver must have set chip->controller beforehand. Otherwise, >> nand_attach() causes NULL pointer dereference. >> >> In fact, the Denali controller knows the number of connected chips >> before calling nand_scan_ident(); if DEVICE_RESET fails, there is no >> chip in that chip select. Then, denali_reset_banks() sets the maxchips >> to the number of detected chips. If no chip is found, it is zero. >> >> The reason of this trick was, as commit f486287d2372 ("mtd: nand: >> denali: fix bank reset function to detect the number of chips") >> explained, nand_scan_ident() issued Set Features (0xEF) command >> to all CS lines, some of which may not be connected with a chip. >> Then, the driver would wait until R/B# response, which never happens. >> >> This problem was solved by commit 107b7d6a7ad4 ("mtd: rawnand: avoid >> setting again the timings to mode 0 after a reset"). In the current >> code, nand_setup_data_interface() is called from nand_scan_tail(), >> which is after the chip detection is done. >> >> Remove the code that is causing NULL pointer dereference. Now, the >> maxchips passed to nand_scan() is the maximum number of chip selects >> supported by the IP (typically 4 or 8). Leave all the chip detection >> process to nand_scan_ident(). >> >> Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> >> --- >> >> drivers/mtd/nand/raw/denali.c | 1 - >> 1 file changed, 1 deletion(-) >> >> diff --git a/drivers/mtd/nand/raw/denali.c b/drivers/mtd/nand/raw/denali.c >> index ca18612..3e4b8e1 100644 >> --- a/drivers/mtd/nand/raw/denali.c >> +++ b/drivers/mtd/nand/raw/denali.c >> @@ -1086,7 +1086,6 @@ static void denali_reset_banks(struct denali_nand_info *denali) >> } >> >> dev_dbg(denali->dev, "%d chips connected\n", i); >> - denali->max_banks = i; > > Shouldn't we instead avoid calling nand_scan() when > denali->max_banks=0? I mean, what's the point of calling this function > if you know for sure it will fail. Right. If no chip is found, it should error out with -ENODEV or something. > Last question: do we still need this denali_reset_banks()? If it's only > about resetting the chip to detect how many are actually present, > that's already done by nand_scan(). I thought this too. Please give me time to answer this question. I need to check the datasheet and test on my boards. If I can remove denali_reset_banks() entirely, it would be the best. Thanks. >> } >> >> static void denali_hw_init(struct denali_nand_info *denali) > -- Best Regards Masahiro Yamada ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/