sam9x5: MTD numbering changed

Thu Nov 2 16:12:17 PDT 2017

Hi all,

Le 02/11/2017 à 18:58, Boris Brezillon a écrit :
> On Thu, 2 Nov 2017 18:36:35 +0100
> Richard Genoud <richard.genoud at gmail.com> wrote:
> 
>> 2017-11-02 16:45 GMT+01:00 Boris Brezillon <boris.brezillon at free-electrons.com>:
>>> On Thu, 02 Nov 2017 16:28:13 +0100
>>> Richard Genoud <richard.genoud at gmail.com> wrote:
>>>  
>>>> +Nicolas
>>>> [its email got lost somehow]
>>>> Le jeudi 02 novembre 2017 à 16:09 +0100, Boris Brezillon a écrit :  
>>>>> On Thu, 02 Nov 2017 15:13:47 +0100
>>>>> Richard Genoud <richard.genoud at gmail.com> wrote:
>>>>>  
>>>>>> Le jeudi 02 novembre 2017 à 13:39 +0100, Boris Brezillon a écrit :  
>>>>>>> +Nicolas
>>>>>>>
>>>>>>> Hi Richard,
>>>>>>>
>>>>>>> On Thu, 02 Nov 2017 12:17:16 +0100
>>>>>>> Richard Genoud <richard.genoud at gmail.com> wrote:
>>>>>>>  
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> I've got an at91sam9g35-cm based board, with 4 partition on the
>>>>>>>> spi-
>>>>>>>> dataflas and 5 partitions on the NAND flash.
>>>>>>>> Before commit 1004a2977bdc ("ARM: dts: at91: Switch to the new
>>>>>>>> NAND
>>>>>>>> bindings"),
>>>>>>>> the NAND partitions were mtd0-4 and spi-dataflash partitions
>>>>>>>> mtd5-
>>>>>>>> 8.
>>>>>>>>
>>>>>>>> Since commit 1004a2977bdc ("ARM: dts: at91: Switch to the new
>>>>>>>> NAND
>>>>>>>> bindings"),
>>>>>>>> the spi-dataflash partitions are discovered before the NAND
>>>>>>>> partitions.
>>>>>>>> So NAND partition became mtd4-8 and spi-dataflash partition
>>>>>>>> mtd0-3.
>>>>>>>>
>>>>>>>> This broke some script that relied on the mtd numbering.
>>>>>>>>
>>>>>>>> Updating those scripts to rely on the mtd device name instead
>>>>>>>> of
>>>>>>>> number is not really a problem. The real problem is when an
>>>>>>>> older
>>>>>>>> script using mtd numbering is run on the new system : I expect
>>>>>>>> dead
>>>>>>>> kittens everywhere !  
>>>>>>>
>>>>>>> Crap! That was one of the thing I was afraid of when changing the
>>>>>>> binding: probe order has an impact on ids assigned to MTD devs,
>>>>>>> and
>>>>>>> since things are not defined at the same place in the DT, it
>>>>>>> changes
>>>>>>> the probe order.
>>>>>>>  
>>>>>>>>
>>>>>>>> So, I'd like to know if there's a way to force the older
>>>>>>>> numbering
>>>>>>>> ?  
>>>>>>>
>>>>>>> Reverting the patches is probably the easiest way (and it's
>>>>>>> easily
>>>>>>> backportable). Now, if we want to switch to the new bindings at
>>>>>>> some
>>>>>>> point we'll need to support DT aliases for mtd devs:
>>>>>>>
>>>>>>> aliases {
>>>>>>>         mtdX = &flashpartN;
>>>>>>>         mtdY = &flashdevM;
>>>>>>> };
>>>>>>>
>>>>>>> The problem with this solution is that it only works if all
>>>>>>> partitions
>>>>>>> are defined in the DT, which is not always the case (they can be
>>>>>>> defined
>>>>>>> on the command line with mtdparts=).  
>>>>>>
>>>>>> Yes, and if they are different from the ones declared in
>>>>>> at91sam9x5cm.dtsi, they are likely defined with mtdparts=, since
>>>>>> AFAIK,
>>>>>> we can't remove a declared partitionning.
>>>>>>
>>>>>> I'll disable the ebi and switch back to the old binding in my dts
>>>>>> for
>>>>>> now.  
>>>>>>>  
>>>>>>>> (I tried poking around the DTS without succès).
>>>>>>>>
>>>>>>>> any idea ?  
>>>>>>>
>>>>>>> I don't have a perfect solution, but the problem you report
>>>>>>> clearly
>>>>>>> shows that relying on MTD numbering is unsafe and should be
>>>>>>> avoided.  
>>>>>>
>>>>>> Clearly, but who doesn't ? ;)
>>>>>>  
>>>>>
>>>>> Just had a lengthy discussion with Alexandre, and he brought a valid
>>>>> point: there has never been any guarantee on MTD numbering. Not only
>>>>> the order of DT nodes have an impact on the probe order, but also the
>>>>> order in which drivers are linked when creating the kernel image. Yes
>>>>> these things usually don't change, but I'm not sure it's a good idea
>>>>> to let user-space apps think it will never change in the future.
>>>>>
>>>>> How about fixing the scripts you were referring to instead of
>>>>> reverting
>>>>> the change? What's the blocking point?  
>>>>
>>>> I already fixed the user-space scripts (and actually, they predate the
>>>> device-tree era, so at that time, relying on MTD numbering wasn't so
>>>> bad :)).
>>>> Anyway, here's the blocking point :
>>>>
>>>> We have firmwares with an embedded script to update our boards. (more
>>>> or less a FW + script in a zip file).
>>>> Those old firmwares are already in the wild and rely on the old MTD
>>>> numbering (yes, that's bad).
>>>>
>>>>
>>>> So, even if all new scripts are corrected in the new firmwares,
>>>> downgrading a board with an old firmware/old script will brick the
>>>> board.
>>>> So I'll have to detect that and forbid downgrading.  
>>>
>>> Not sure what you refer to when you're talking about 'FW', but I'd
>>> expect it to contain a kernel [+ a dtb] + a rootfs, so if you're using
>>> an old "FW+update-script", you will still have the old numbering and
>>> the old script will work just fine, and if you switch to a newer version
>>> of the "FW+update-script" based on a 4.13 kernel+dtb you will anyway
>>> have the updated script. Am I missing something?  
>>
>> Ok, let's have an example.
>> New firmware : kernel
>> 4.14+dtb+rootfs+uboot+uboot_env+at91bootstrap+new update script.
>> Old firmware : kernel 4.11 + dtb + rootfs + uboot +
>> uboot_env+at91bootstrap+old update script.
>>
>> Let's suppose there's the new firmware on the board (let's call it
>> 4.14 firmware) :
>> MTD0 is the dataflash partition 0 (at91bootstrap)
>> MTD1 is the dataflash part1 (uboot)
>> MTD2 is the dataflash part2 (ubootenv)
>> MTD3 is the dataflash part3 (free space)
>> MTD4 is the NAND part 0 (dtb)
>> MTD5 is the NAND part 1 (kernel)
>> MTD6 is the NAND part 2 (UBI) <- in this one, there're 3 ubifs volumes
>> : rootfs/rootfs2/data
>>
>> The thing is, the update script runs in user-space and updates the
>> kernel, dtb, uboot, bootstrap and flashes the rootfs2.
>> (on next boot, the rootfs2/rootfs will be atomically swapped)
>> So, when downgrading, the old update script is executed on the board,
>> under the 4.14 firmware, thinking that MTD0 is the NAND part0 (dtb),
>> while it is actually the dataflash part0 (bootstrap).
>> => the bootstrap is erased and replaced by the dtb (well, actually, it  
>> won't even be like that since there will be a mismatch between
>> nandwrite/flash_part).
> 
> Can you re-generate zip archives for old versions? If you can maybe it
> would be simpler to just embed the new update script (assuming this
> script works fine with both old and new FW) in old FW archives.
>

No sure, I've totally understand the issue, if not sorry for the noise!

Would it be possible to use some udev rules to create some symbolic links
in /dev based on ATTRS{name}, the mtd partition names?

something like:
/dev/mtd-at91bootstrap -> /dev/mtd0
/dev/mtd-uboot -> /dev/mtd1
/dev/mtd-ubootenv -> /dev/mtd2
/dev/mtd-free -> /dev/mtd3
/dev/mtd-dtb -> /dev/mtd4
/dev/mtd-kernel -> /dev/mtd5
/dev/mtd-ubi -> /dev/mtd6

Then if the new update script only uses symbolic names, it should work with
both linux 4.11 and 4.14 kernels.

But I think I didn't totally understand why/how the old update script would
be run under the new kernel :p

Best regards,

Cyrille

>>
>> It's not a huge big deal since I have another way to flash all
>> partitions from uboot, but this can't be done remotely.
>>
>>
>>>  
>>>>
>>>> That's not the end of the world, but if I can find a trick to prevent
>>>> it, I'll be happier !
>>>>
>>>>  
> 
> 
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