Re: MTD: How to get actual image size from MTD partition

Hi,


On Fri, 29 Oct 2021 at 22:18, Ezequiel Garcia
[off-list ref] wrote:

On Fri, 29 Oct 2021 at 13:13, Pintu Agarwal [off-list ref] wrote:

quoted

Hi All,

On Mon, 30 Aug 2021 at 21:28, Pintu Agarwal [off-list ref] wrote:

quoted

On Sun, 22 Aug 2021 at 19:51, Ezequiel Garcia
[off-list ref] wrote:

quoted

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.)

Yes, this is what we are also doing exactly.
But, currently we are running into this issue right now:
1) The FOTA update is failing if we use static volume (building and
flashing the static image is fine)
    It works fine, if we use the volume as dynamic (even for RO type
with squashfs)
    But with dynamic type we end up using the entire volume as image size.

So, we wanted to know if there is any other way to get volume image
size at runtime ?


Thanks,
Pintu

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