Re: [PATCH 1/3] mtd: sharpsl: add sharpslpart MTD partition parser

[Boris Brezillon]

Hi Andrea,

On Tue, 18 Apr 2017 10:58:02 +0200
Andrea Adami <andrea.adami@xxxxxxxxx> wrote:

> Boris,
> thanks for having read it.
> 
> On Mon, Apr 17, 2017 at 4:44 PM, Boris Brezillon <
> boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
> > Marek, Andrea,
> >
> > Before we even start discussing minor improvements (like coding style),
> > I'd like to discuss the sharp FTL and partition table format, and
> > decide whether we want to have such an old FTL included in the kernel.
> >
> > Actually, that's the very reason I asked Andrea to post his series as
> > soon as possible even if some things were not perfect.
> >
> > I'll try to document myself on the on-flash format of the FTL and
> > partition table before giving a definitive opinion, but I have the
> > feeling this ancient FTL is not 100% safe, and by accepting an old
> > (maybe unreliable?) FTL we are setting a precedent, and refusing other
> > (broken) proprietary/vendor FTLs will be almost impossible after that.
> >
> > Maybe I'm wrong, and this FTL is really safe and can be used on other
> > devices, that's why I'd like to understand how it works before giving
> > my opinion.
> >  
> 
> I can't judge the work done by ARM/Sharp/Linaro 15 years ago...
> I have only seen this on this PXA Sharp Zaurus SL series.
> 
> For the records, these are the GPL 2.4 sources
> https://github.com/LinuxPDA/Sharp_FTL_2.4.20

That's exactly why I said someone should review it, and I'm not talking
about the code itself, but the FTL+partition-table format.

> 
> >
> > On Sun, 16 Apr 2017 18:07:47 +0200
> > Marek Vasut <marek.vasut@xxxxxxxxx> wrote:
> >  
> >> On 04/15/2017 10:11 PM, Andrea Adami wrote:  
> >> > The Sharp SL Series (Zaurus) PXA handhelds have 16/64/128M of NAND  
> flash
> >> > and share the same layout of the first 7M partition, managed by Sharp  
> FTL.
> >> >
> >> > The purpose of this self-contained patch is to add a common parser and
> >> > remove the hardcoded sizes in the board files (these devices are not  
> yet
> >> > converted to devicetree).
> >> > Users will have benefits because the mtdparts= tag will not be  
> necessary
> >> > anymore and they will be free to repartition the little sized flash.
> >> >
> >> > The obsolete bootloader can not pass the partitioning info to modern
> >> > kernels anymore so it has to be read from flash at known logical  
> addresses.
> >> > (see http://www.h5.dion.ne.jp/~rimemoon/zaurus/memo_006.htm )  
> >
> >
> > Okay, IMO that's not really a good argument to support this partition
> > parser. As Richard and I already mentioned several times, if your
> > bootloader is not capable of passing valid mtdparts= you can hardcode
> > it in the kernel using a default CMDLINE.
> >  
> We are simply trying to restore the ability for the kernel to read the
> mtdparts.
> 
> There are around many kernels and distros (OpenEmbedded, Arch, older 2.x
> stuff).
> Some do require to repartition: how can I know how other people partitioned?

By using a 2nd stage bootloader, as you did.

> 
> > Now, I understand that you want to support multiple devices with the
> > same kernel, and having this partition parser would simplify your job.
> > I also know you are developing a 2nd stage bootloader (based on
> > kexec+a minimal initramfs) to address the limitations of the
> > non-upgradable 1st stage bootloader.
> >
> > According to the rest of the description, you already have user-space
> > tools to manipulate the partition-table and those are aware of the FTL
> > layer, so I think you have all the basic blocks to get rid of this
> > in-kernel implementation.
> >
> > All you need is a way to extract the partitions from the 2nd stage
> > bootloader (using some tools you have in your initramfs) and build the
> > according mtdparts= parameter to pass to kexec when booting the real
> > kernel (the one used by the system).
> >  
> 
> We have patched kexecboot long ago to do that.
> https://github.com/kexecboot/kexecboot/blob/master/machine/zaurus.c

Okay, so that's what I initially understood. You already have a fully
functional user-space solution.

> 
> This is done with a special kernel (linux-kexecboot) embedding the minimal
> cpio and acting as 2nd stage bootloader.
> It passes the mtdparts found in cmdline and does the extra trick of reading
> it from mtd1 for zaurus.
> You can even customize the cmdline in /boot/boot.cfg and hack the mtdparts
> there.
> 
> Neverthless, what you don't seem to understand is that I cannot force
> people to use kexecboot or to customize cmdline parts as I like...
> I do just build kernels and images for testing...I maintain the OE build
> infrastructure, not one distro.

Well, you can say "if you want to use a mainline kernel, stop using
this sharp FTL+partition-table and start using a 2nd stage
bootloader like kexecboot". What do they use right now to boot a new
kernel (newer than the 2.4 one)?

> 
> > You tried to explain why it was not doable, but I still don't see
> > why.
> > You first said that not all systems had CONFIG_MTD_CMDLINE_PARTS
> > enabled and that some people were booting distro kernels. Honestly, I
> > think you have more chances to have CONFIG_MTD_CMDLINE_PARTS in those
> > distro kernels than having your custom FTL enabled.
> > Then you tried to explain that with the user-space only solution you
> > wouldn't be able to support systems where the user had resized the
> > partitions with the nandlogical tool, and I still don't see why. Maybe
> > you can give more details to explain why this is impossible.  
> 
> I don't understand why people should get crazy with the different mtdparts=
> for each model.

So you agree that passing partition info through the cmdline is a good
solution?

> IMHO we are restoring a basic functionality, anything weird.

Are you talking about sharp FTL+part-table or the cmdline mtdparts=
parameter?

> 
> >
> > Just going through all these details to say that, IMO, we should only
> > consider inclusion of this feature if we think it's safe, because I
> > think all that is done here can be done from user-space.
> >  
> Read only is safe.
> 

This is a lie. AFAIU, you have all the necessary tools to update the
partition table from user-space, so even if you only have read-only
support in the kernel, one can corrupt it from userspace, and the
kernel may not be able to recover from this corruption.

Honestly, if we want to support this FTL+partition-table-format in the
kernel, I'd recommend that we add RW support, otherwise you'll keep
having those external tools.