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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Sat, 25 Aug 2018 00:04:43 +0900
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Subject: Re: [PATCH] mtd: rawnand: denali: do not pass zero maxchips to nand_scan()
To:     Boris Brezillon <boris.brezillon@bootlin.com>
Cc:     linux-mtd <linux-mtd@lists.infradead.org>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Marek Vasut <marek.vasut@gmail.com>,
        Brian Norris <computersforpeace@gmail.com>,
        Richard Weinberger <richard@nod.at>,
        David Woodhouse <dwmw2@infradead.org>
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Hi Boris,

2018-08-24 21:55 GMT+09:00 Boris Brezillon <boris.brezillon@bootlin.com>:
> Hi Masahiro,
>
> On Tue, 21 Aug 2018 17:23:19 +0900
> Masahiro Yamada <yamada.masahiro@socionext.com> 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@socionext.com>
>> ---
>>
>>  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