linux-kernel - Re: [PATCH] Fix writing mtdoops to nand flash.

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Message-ID: <CAP+RiCCHV3V82695WPv9nQ18Ascx_WeCB+=3er4S0Zn4YK+K4g@mail.gmail.com>
Date:   Mon, 30 Oct 2017 07:46:18 -0500
From:   Brent Taylor <motobud@...il.com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix writing mtdoops to nand flash.

Hi Bors, thanks for your quick reply.

On Mon, Oct 30, 2017 at 3:23 AM, Boris Brezillon
<boris.brezillon@...e-electrons.com> wrote:
> Hi Brent,
>
> Subject should be prefixed by "mtd: nand: ", so
>
> "mtd: nand: Fix writing mtdoops to nand flash"

Oops, yes, will fix.

>
> On Sun, 29 Oct 2017 23:23:43 -0500
> motobud@...il.com wrote:
>
>> From: Brent Taylor <motobud@...il.com>
>>
>> When mtdoops calls mtd_panic_write, it eventually calls
>> panic_nand_write in nand_base.c.  In order to properly
>> wait for the nand chip to be ready in panic_nand_wait,
>> the chip must first be selected.
>>
>> When using the atmel nand flash controller, a panic
>> would occur due to a NULL pointer exception.
>>
>> Signed-off-by: Brent Taylor <motobud@...il.com>
>> ---
>>  drivers/mtd/nand/nand_base.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
>> index 12edaae17d81..0a8058a66d93 100644
>> --- a/drivers/mtd/nand/nand_base.c
>> +++ b/drivers/mtd/nand/nand_base.c
>> @@ -2802,9 +2802,14 @@ static int panic_nand_write(struct mtd_info *mtd, loff_t to, size_t len,
>>       struct mtd_oob_ops ops;
>>       int ret;
>>
>> +     int chipnr = (int)(to >> chip->chip_shift);
>> +     chip->select_chip(mtd, chipnr);
>> +
>>       /* Wait for the device to get ready */
>>       panic_nand_wait(mtd, chip, 400);
>>
>> +     chip->select_chip(mtd, -1);
>> +
>
> Duh! Looks like a piece of code that is never tested. Did you face the
> problem or did you find out by inspecting the code?

I was playing with another driver on an atmel development board
(at91sam9m10g45ek) and caused a panic with mtdoops enabled.  While
writing the mtdoops to nand, another panic occurred which caused a
storm of panics to generated.

>
> Anyway, I fear the atmel driver is not the only one to rely on the chip
> to be selected when ->dev_ready() is called so this should definitely
> be fixed. Also, we should probably move the ->select_chip() and
> panic_nand_wait() calls after panic_nand_get_device(), and I don't
> think we need to unselect the chip (it will be taken care of by
> nand_do_write_ops()).
>
>>       /* Grab the device */
>>       panic_nand_get_device(chip, mtd, FL_WRITING);
>>
>

After looking at this closer, a panic could happen at any point correct?  If
that's the case, the kernel could have been in the middle of a nand read/write
operation (which may or may not be on the same chip).  Would the chip the
mtdoops is being written to need to be reset?  I haven't drilled down into the
nand_reset function yet, but can that be called in a "panic" state?