Re: [RFC PATCH 08/20] boot: image-loader: add UBI volume backend

[Daniel Golle <daniel@makrotopia.org>]

On Mon, Feb 23, 2026 at 10:30:03PM +0300, Mikhail Kshevetskiy wrote:
> 
> On 2/23/26 22:24, Daniel Golle wrote:
> > On Mon, Feb 23, 2026 at 10:51:09AM -0700, Simon Glass wrote:
> >> Hi Daniel,
> >>
> >> On Thu, 19 Feb 2026 at 09:51, Daniel Golle <daniel@makrotopia.org> wrote:
> >>> On Thu, Feb 19, 2026 at 06:09:27AM -0700, Simon Glass wrote:
> >>>> Hi Daniel,
> >>>>
> >>>> On Mon, 16 Feb 2026 at 14:22, Daniel Golle <daniel@makrotopia.org> wrote:
> >>>>> Add a UBI volume storage backend for the image_loader framework.
> >>>>>
> >>>>> image_loader_init_ubi() takes a volume name, ensures the UBI device
> >>>>> is attached (auto-attaching if needed), and installs a .read() callback
> >>>>> wrapping ubi_volume_read().
> >>>>>
> >>>>> Auto-attach works by scanning the device tree for the first MTD
> >>>>> partition with compatible = "linux,ubi", then calling ubi_part() on
> >>>>> that partition. The partition name is resolved using the same
> >>>>> precedence as the MTD partition parser: the "label" property first,
> >>>>> then "name", then the node name. Since U-Boot only supports a single
> >>>>> attached UBI device at a time, only the first matching partition is
> >>>>> used. If a UBI device is already attached, the auto-attach step is
> >>>>> skipped.
> >>>>>
> >>>>> UBI handles bad-block management and wear leveling internally, so the
> >>>>> read callback is a straightforward passthrough. Note that
> >>>>> ubi_volume_read() returns positive errno values; the wrapper negates
> >>>>> them for the image_loader convention.
> >>>>>
> >>>>> Gated by CONFIG_IMAGE_LOADER_UBI (depends on CMD_UBI && IMAGE_LOADER).
> >>>>>
> >>>>> Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> >>>>> ---
> >>>>>  boot/Kconfig            |   8 +++
> >>>>>  boot/Makefile           |   1 +
> >>>>>  boot/image-loader-ubi.c | 112 ++++++++++++++++++++++++++++++++++++++++
> >>>>>  include/image-loader.h  |  13 +++++
> >>>>>  4 files changed, 134 insertions(+)
> >>>>>  create mode 100644 boot/image-loader-ubi.c
> >>>>>
> >>>>> diff --git a/boot/Kconfig b/boot/Kconfig
> >>>>> index 23848a0f57e..89832014af6 100644
> >>>>> --- a/boot/Kconfig
> >>>>> +++ b/boot/Kconfig
> >>>>> @@ -1203,6 +1203,14 @@ config IMAGE_LOADER_MTD
> >>>>>           parallel NAND, etc.) using the image_loader framework.
> >>>>>           NAND bad blocks are skipped transparently.
> >>>>>
> >>>>> +config IMAGE_LOADER_UBI
> >>>>> +       bool "UBI volume backend for image loader"
> >>>>> +       depends on IMAGE_LOADER && CMD_UBI
> >>>>> +       help
> >>>>> +         Allows loading images from UBI volumes using the image_loader
> >>>>> +         framework. Auto-attaches the UBI device from the device tree
> >>>>> +         if not already attached.
> >>>>> +
> >>>>>  config DISTRO_DEFAULTS
> >>>>>         bool "(deprecated) Script-based booting of Linux distributions"
> >>>>>         select CMDLINE
> >>>>> diff --git a/boot/Makefile b/boot/Makefile
> >>>>> index 1dde16db694..7d1d4a28106 100644
> >>>>> --- a/boot/Makefile
> >>>>> +++ b/boot/Makefile
> >>>>> @@ -76,6 +76,7 @@ obj-$(CONFIG_$(PHASE_)BOOTMETH_ANDROID) += bootmeth_android.o
> >>>>>  obj-$(CONFIG_IMAGE_LOADER) += image-loader.o
> >>>>>  obj-$(CONFIG_IMAGE_LOADER_BLK) += image-loader-blk.o
> >>>>>  obj-$(CONFIG_IMAGE_LOADER_MTD) += image-loader-mtd.o
> >>>>> +obj-$(CONFIG_IMAGE_LOADER_UBI) += image-loader-ubi.o
> >>>>>
> >>>>>  obj-$(CONFIG_$(PHASE_)BOOTMETH_VBE_ABREC) += vbe_abrec.o vbe_common.o
> >>>>>  obj-$(CONFIG_$(PHASE_)BOOTMETH_VBE_ABREC_FW) += vbe_abrec_fw.o
> >>>>> diff --git a/boot/image-loader-ubi.c b/boot/image-loader-ubi.c
> >>>>> new file mode 100644
> >>>>> index 00000000000..64901a13378
> >>>>> --- /dev/null
> >>>>> +++ b/boot/image-loader-ubi.c
> >>>>> @@ -0,0 +1,112 @@
> >>>>> +// SPDX-License-Identifier: GPL-2.0+
> >>>>> +/*
> >>>>> + * UBI volume backend for image_loader
> >>>>> + *
> >>>>> + * Copyright (C) 2026 Daniel Golle <daniel@makrotopia.org>
> >>>>> + */
> >>>>> +
> >>>>> +#include <dm/ofnode.h>
> >>>>> +#include <image-loader.h>
> >>>>> +#include <log.h>
> >>>>> +#include <malloc.h>
> >>>>> +#include <mtd.h>
> >>>>> +#include <ubi_uboot.h>
> >>>>> +
> >>>>> +struct image_loader_ubi_priv {
> >>>>> +       char *vol_name;
> >>>>> +};
> >>>>> +
> >>>>> +/**
> >>>>> + * ubi_auto_attach() - attach UBI if not already attached
> >>>>> + *
> >>>>> + * If no UBI device is currently attached, walk the device tree for the
> >>>>> + * first MTD partition node with compatible = "linux,ubi", find the
> >>>>> + * corresponding MTD device by matching flash_node, and attach UBI to
> >>>>> + * it via ubi_part_from_mtd().
> >>>>> + *
> >>>>> + * Since U-Boot only supports a single attached UBI device at a time,
> >>>>> + * only the first matching partition is used.
> >>>>> + *
> >>>>> + * Return: 0 on success or if already attached, negative errno on failure
> >>>>> + */
> >>>>> +static int ubi_auto_attach(void)
> >>>>> +{
> >>>>> +       struct mtd_info *mtd;
> >>>>> +       ofnode node;
> >>>>> +
> >>>>> +       /* Already attached? */
> >>>>> +       if (ubi_devices[0])
> >>>>> +               return 0;
> >>>>> +
> >>>>> +       mtd_probe_devices();
> >>>> This should be handled by your new ubi driver for your uclass.
> >>>>
> >>>>> +
> >>>>> +       ofnode_for_each_compatible_node(node, "linux,ubi") {
> >>>>> +               mtd_for_each_device(mtd) {
> >>>>> +                       if (ofnode_equal(mtd->flash_node, node))
> >>>>> +                               goto found;
> >>>>> +               }
> >>>>> +       }
> >>>> Eek this is really strange. There should be a device so you can use
> >>>> uclass_first_device(UCLASS_...).
> >>> I carefully considered turning UBI devices into a UCLASS, but it would
> >>> be a huge major rewrite of how UBI works in U-Boot, and while that would
> >>> be a good thing to do, I consider far beyond the scope of supporting a
> >>> boot method for OpenWrt.
> >> Oh, I just assumed it supports driver model. But surely it must have a
> >> UCLASS_BLK device?
> > Sadly no. Neither UBI devices nor UBI volumes are block storage devices.
> >
> > Every UBI device does have a UCLASS_MTD parent device, but that
> > obviously also doesn't uniquely identify the partition on the flash chip
> > which is used as UBI device -- there can be (and sometimes are, for
> > dual-boot/redundancy reasons) multiple UBI devices in different
> > partitions on the same flash.
> >
> > What I ended up doing now is to move more of the UBI detection and
> > enumeration logic into ubi_bootdev.c and have imagemap-ubi.c rely
> > on getting the MTD device and partition index from there.
> 
> There are UBI based block storage emulation, see CONFIG_UBI_BLOCK
> commits: 
> * 9daad11ad178646c288aca3615a7ba1e6039aed3 ("drivers: introduce UBI
> block abstraction")
> * aa5b67ce226267440e64fadc57d3a21e5842027c ("disk: support UBI partitions")

Yes, but that also only works once the UBI device is already attached.
Before that, and for any UBI device not currently attached, there aren't
any UBI_BLOCK devices. However, in both cases we'd also need to scan
them for volumes to be presented as potential bootdevs.