linux-kernel - Re: [PATCH 2/2] arch: arm64: configs: Enable UBI and UBIFS

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Date:   Mon, 26 Jun 2023 10:38:30 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     "Lee, Kah Jing" <kah.jing.lee@...el.com>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] arch: arm64: configs: Enable UBI and UBIFS

On 26/06/2023 06:16, Lee, Kah Jing wrote:
>> -----Original Message-----
>> From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>> Sent: Saturday, 24 June, 2023 3:30 PM
>> To: Lee, Kah Jing <kah.jing.lee@...el.com>; Dinh Nguyen
>> <dinguyen@...nel.org>; Rob Herring <robh+dt@...nel.org>; Krzysztof
>> Kozlowski <krzysztof.kozlowski+dt@...aro.org>; Conor Dooley
>> <conor+dt@...nel.org>; Catalin Marinas <catalin.marinas@....com>; Will
>> Deacon <will@...nel.org>
>> Cc: linux-kernel@...r.kernel.org
>> Subject: Re: [PATCH 2/2] arch: arm64: configs: Enable UBI and UBIFS
>>
>> On 24/06/2023 05:42, Lee, Kah Jing wrote:
>>>>>> So you miss init ramdisk.
>>>>> Currently we are using the bootargs to mount the rootfs from QSPI
>>>>> NOR
>>>> flash:
>>>>> [    0.000000] Kernel command line: earlycon panic=-1 ubi.mtd=1
>>>> root=ubi0:rootfs rootfstype=ubifs rw rootwait
>>>>> Is it possible to mount the ubifs rootfs with the ubifs=m config during
>> boot?
>>>>
>>>> I think yes. rootfs devices are for example modules, so filesystem
>>>> can be as well.
>>> Was going through mtd ubifs page -
>>> http://www.linux-mtd.infradead.org/faq/ubifs.html
>>> Quoted: 'In order to mount UBIFS as the root file system, you have to
>>> compile UBIFS into the kernel (instead of compiling it as a kernel
>>> module) and specify proper kernel boot arguments and make the kernel
>> mount UBIFS on boot.'
>>
>> Why? Module loaded by initramfs would also understand cmdline arguments,
>> right?
> The suggestion is to use initramfs for rootfs -> remount UBIFS as chroot?
> The concern is additional initrd image and steps to store in the
> limited NOR flash (256MB, Boot data + Uboot - ~66MB, UBIFS image - ~88MB, 
> kernel.itb - ~10MB = 164MB).
> With the mounting Rootfs from UBIFS volume, we can skip the initrd step, and
> save some space for the user operations. 
> Let me know if I understands that correctly.

arm64 defconfig creates huge development config for all platforms, so
why would you ever use it in resource-constrained system? It would
barely fit. defconfig modules take 50 MB alone and you don't need most
of them.

I think you misunderstood the purpose of this defconfig and now try to
apply some arguments for different use cases.

Best regards,
Krzysztof