MTD: How to get actual image size from MTD partition

[Ezequiel Garcia]

On Fri, 29 Oct 2021 at 13:13, Pintu Agarwal <pintu.ping at gmail.com> wrote:
>
> Hi All,
>
> On Mon, 30 Aug 2021 at 21:28, Pintu Agarwal <pintu.ping at gmail.com> wrote:
> >
> > On Sun, 22 Aug 2021 at 19:51, Ezequiel Garcia
> > <ezequiel at vanguardiasur.com.ar> wrote:
> >
> > > In other words, IMO it's best to expose the NAND through UBI
> > > for both read-only and read-write access, using a single UBI device,
> > > and then creating UBI volumes as needed. This will allow UBI
> > > to spread wear leveling across the whole device, which is expected
> > > to increase the flash lifetime.
> > >
> > > For instance, just as some silly example, you could have something like this:
> > >
> > >                                | RootFS SquashFS  |
> > >                                | UBI block        | UBIFS User R-W area
> > > ------------------------------------------------------------------------
> > > Kernel A | Kernel B | RootFS A | RootFS B         | User
> > > ------------------------------------------------------------------------
> > >                                  UBIX
> > > ------------------------------------------------------------------------
> > >                                  /dev/mtdX
> > >
> > > This setup allows safe kernel and rootfs upgrading. The RootFS is read-only
> > > via SquashFS and there's a read-write user area. UBI is supporting all
> > > the volumes, handling bad blocks and wear leveling.
> > >
> > Dear Ezequiel,
> > Thank you so much for your reply.
> >
> > This is exactly what we are also doing :)
> > In our system we have a mix of raw and ubi partitions.
> > The ubi partitioning is done almost exactly the same way.
> > Only for the rootfs (squashfs) I see we were using /mtd/block<id> to
> > mount the rootfs.
> > Now, I understood we should change it to use /dev/ubiblock<id>
> > This might have several benefits, but one most important could be,
> > using ubiblock can handle bad-blocks/wear-leveling automatically,
> > whereas mtdblocks access the flash directly ?
> > I found some references for these..
> > So, this seems good for my proposal.
> >
> > Another thing that is still open for us is:
> > How do we calculate the exact image size from a raw mtd partition ?
> > For example, support for one of the raw nand partitions, the size is
> > defined as 15MB but we flash the actual image of size only 2.5MB.
> > So, in the runtime how to determine the image size as ~2.5MB (at least
> > roughly) ?
> > Is it still possible ?
> >
>
> I am happy to inform you that using "ubiblock" for squashfs mounting
> seems very helpful for us.
> We have seen almost the double performance boost when using ubiblock
> for rootfs as well as other read-only volume mounting.
>
> However, we have found few issues while defining the read only volume as STATIC.
> With static volume we see that OTA update is failing during "fsync".
> That is ota_fsync is failing from here:
> https://gerrit.pixelexperience.org/plugins/gitiles/bootable_recovery/+/ff6df890a2a01bf3bf56d3f430b17a5ef69055cf%5E%21/otafault/ota_io.cpp
> int status = fsync(fd);
> if (status == -1 && errno == EIO)
> *
> { have_eio_error = true; }
> *
> return status;
> }
>
> Is this the known issue with static volume?
>

I don't know exactly how you are updating your volume,
the right way is using UBI_IOCVOLUP.

See http://www.linux-mtd.infradead.org/doc/ubi.html#L_volupdate

If you google around I'm sure you'll find some articles about this,
but I'm not sure if they'll go into details and subtleties.

There are probably a few different ways to do firmware upgrade
when you are on top of static volumes (and you want to be on top
of static volumes if it's read-only, because AFAIK they give you an
extra data-integrity guarantee).

One way, would be to have two static volumes A/B. The system
uses normally the A volume, and then you doUBI_IOCVOLUP
(or ubiupdatevol) to update the B volume. After the update is succesful
you run the atomic volume rename and flip A->B, B->A.

(If you don't have enough space to hold two A/B volumes....
 ... you'll have to find some other solution, I have no idea about that.)

Hope it helps,
Eze