[PATCH] mtd: rawnand: Do not check for bad block if bbt is unavailable

[Manivannan Sadhasivam]

Hi Miquel, 

On 2 February 2021 1:44:59 PM IST, Miquel Raynal <miquel.raynal at bootlin.com> wrote:
>Hi Manivannan,
>
>Manivannan Sadhasivam <manivannan.sadhasivam at linaro.org> wrote on Tue,
>2 Feb 2021 09:46:14 +0530:
>
>> Hi,
>> 
>> On Mon, Feb 01, 2021 at 03:18:24PM +0100, Miquel Raynal wrote:
>> > Hi Manivannan,
>> > 
>> > Manivannan Sadhasivam <manivannan.sadhasivam at linaro.org> wrote on
>Sat,
>> > 30 Jan 2021 09:24:12 +0530:
>> >   
>> > > The bbt pointer will be unavailable when NAND_SKIP_BBTSCAN option
>is
>> > > set for a NAND chip. The intention is to skip scanning for the
>bad
>> > > blocks during boot time.  
>> > 
>> > I don't have the same understanding: this flag skips the bad block
>> > table scan, not the bad block scan. We do want to scan all the
>devices
>> > in order to construct a RAM based table.
>> >   
>> > > However, the MTD core will call
>> > > _block_isreserved() and _block_isbad() callbacks unconditionally
>for
>> > > the rawnand devices due to the callbacks always present while
>collecting
>> > > the ecc stats.
>> > > 
>> > > The _block_isreserved() callback for rawnand will bail out if bbt
>> > > pointer is not available. But _block_isbad() will continue
>without
>> > > checking for it. So this contradicts with the NAND_SKIP_BBTSCAN
>option
>> > > since the bad block check will happen anyways (ie., not much
>difference
>> > > between scanning for bad blocks and checking each block for bad
>ones).
>> > > 
>> > > Hence, do not check for the bad block if bbt pointer is
>unavailable.  
>> > 
>> > Not checking for bad blocks at all feels insane. I don't really get
>the
>> > scope and goal of such change?
>> >   
>> 
>> The issue I encountered is, on the Telit FN980 device one of the
>> partition seems to be protected. So trying to read the bad blocks in
>> that partition makes the device to reboot during boot.
>
>o_O
>
>Reading a protected block makes the device to reboot?
>
>What is the exact device? Can you share the datasheet? Is this behavior
>expected? Because it seems really broken to me, a read should not
>trigger *anything* that bad.
>

I got more information from the vendor, Telit. The access to the 3rd partition is protected by Trustzone and any access in non privileged mode (where Linux kernel runs) causes kernel panic and the device reboots. 

>> There seems to be no flag passed by the parser for this partition. So
>> the only way I could let the device to boot is to completely skip the
>> bad block check.
>
>We do have a "lock" property which informs the host to first unlock the
>device, would this help? Is this locking reversible?
>
>> AFAIK, MTD core only supports checking for the reserved blocks to be
>> used for BBM and there is no way to check for a reserved partition
>like
>> this.
>
>It sounds like a chip specificity/bug, would it make sense to add a
>specific vendor implementation for that?
>

So looks like this is a vendor quirk but this case might arise in future for other platforms as well. 

Thanks, 
Mani

>> I agree that skipping bad block check is not a sane way but I don't
>know
>> any other way to handle this problem.
>> 
>> Thanks,
>> Mani
>> 
>
>Thanks,
>Miquèl

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