Re: [PATCH] mtd: rawnand: denali: do not pass zero maxchips to nand_scan()

Miquel Raynal
2018-08-21T08:42:39Z

Hi Masahiro,

Masahiro Yamada <yamada.masahiro@socionext.com> wrote on Tue, 21 Aug
2018 17:23:19 +0900:

> 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@socionext.com>

Thanks for the fix.

Actually the docg4 driver is getting removed and the special handling
in nand_scan() about having 0 maxchips will also disappear.
Nonetheless, I think the below code was buggy and I would like to apply
the fix anyway.

> ---
>
> 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;
> }
>
> static void denali_hw_init(struct denali_nand_info *denali)

Thanks,
Miquèl