bootstd: doc: Add documentation

Add documentation for this feature, including the commands and full
devicetree bindings.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 638 insertions, 0 deletions

diff --git a/doc/develop/bootstd.rst b/doc/develop/bootstd.rst
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+.. SPDX-License-Identifier: GPL-2.0+:
+
+U-Boot Standard Boot
+====================
+
+Introduction
+------------
+
+Standard boot provides a built-in way for U-Boot to automatically boot
+an Operating System without custom scripting and other customisation. It
+introduces the following concepts:
+
+   - bootdev  - a device which can hold or access a distro (e.g. MMC, Ethernet)
+   - bootmeth - a method to scan a bootdev to find bootflows (e.g. distro boot)
+   - bootflow - a description of how to boot (provided by the distro)
+
+For Linux, the distro (Linux distribution, e.g. Debian, Fedora) is responsible
+for creating a bootflow for each kernel combination that it wants to offer.
+These bootflows are stored on media so they can be discovered by U-Boot. This
+feature is typically called `distro boot` (see :doc:`distro`) because it is
+a way for distributions to boot on any hardware.
+
+Traditionally U-Boot has relied on scripts to implement this feature. See
+disto_boodcmd_ for details. This is done because U-Boot has no native support
+for scanning devices. While the scripts work remarkably well, they can be hard
+to understand and extend, and the feature does not include tests. They are also
+making it difficult to move away from ad-hoc CONFIGs, since they are implemented
+using the environment and a lot of #defines.
+
+Standard boot is a generalisation of distro boot. It provides a more built-in
+way to boot with U-Boot. The feature is extensible to different Operating
+Systems (such as Chromium OS) and devices (beyond just block and network
+devices). It supports EFI boot and EFI bootmgr too.
+
+
+Bootflow
+--------
+
+A bootflow is a file that describes how to boot a distro. Conceptually there can
+be different formats for that file but at present U-Boot only supports the
+BootLoaderSpec_ format. which looks something like this::
+
+   menu autoboot Welcome to Fedora-Workstation-armhfp-31-1.9. Automatic boot in # second{,s}. Press a key for options.
+   menu title Fedora-Workstation-armhfp-31-1.9 Boot Options.
+   menu hidden
+
+   label Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
+       kernel /vmlinuz-5.3.7-301.fc31.armv7hl
+       append ro root=UUID=9732b35b-4cd5-458b-9b91-80f7047e0b8a rhgb quiet LANG=en_US.UTF-8 cma=192MB cma=256MB
+       fdtdir /dtb-5.3.7-301.fc31.armv7hl/
+       initrd /initramfs-5.3.7-301.fc31.armv7hl.img
+
+As you can see it specifies a kernel, a ramdisk (initrd) and a directory from
+which to load devicetree files. The details are described in disto_boodcmd_.
+
+The bootflow is provided by the distro. It is not part of U-Boot. U-Boot's job
+is simply to interpret the file and carry out the instructions. This allows
+distros to boot on essentially any device supported by U-Boot.
+
+Typically the first available bootflow is selected and booted. If that fails,
+then the next one is tried.
+
+
+Bootdev
+-------
+
+Where does U-Boot find the media that holds the operating systems? That is the
+job of bootdev. A bootdev is simply a layer on top of a media device (such as
+MMC, NVMe). The bootdev accesses the device, including partitions and
+filesystems that might contain things related to an operating system.
+
+For example, an MMC bootdev provides access to the individual partitions on the
+MMC device. It scans through these to find filesystems, then provides a list of
+these for consideration.
+
+
+Bootmeth
+--------
+
+Once the list of filesystems is provided, how does U-Boot find the bootflow
+files in these filesystems. That is the job of bootmeth. Each boot method has
+its own way of doing this.
+
+For example, the distro bootmeth simply looks through the provided filesystem
+for a file called `extlinux/extlinux.conf`. This files constitutes a bootflow.
+If the distro bootmeth is used on multiple partitions it may produce multiple
+bootflows.
+
+Note: it is possible to have a bootmeth that uses a partition or a whole device
+directly, but it is more common to use a filesystem.
+
+
+Boot process
+------------
+
+U-Boot tries to use the 'lazy init' approach whereever possible and distro boot
+is no exception. The algorithm is::
+
+   while (get next bootdev)
+      while (get next bootmeth)
+          while (get next bootflow)
+              try to boot it
+
+So U-Boot works its way through the bootdevs, trying each bootmeth in turn to
+obtain bootflows, until it either boots or exhausts the available options.
+
+Instead of 500 lines of #defines and a 4KB boot script, all that is needed is
+the following command::
+
+   bootflow scan -lb
+
+which scans for available bootflows, optionally listing each find it finds (-l)
+and trying to boot it (-b).
+
+
+Controlling ordering
+--------------------
+
+Several options are available to control the ordering of boot scanning:
+
+
+boot_targets
+~~~~~~~~~~~~
+
+This environment variable can be used to control the list of bootdevs searched
+and their ordering, for example::
+
+   setenv boot_targets "mmc0 mmc1 usb pxe"
+
+Entries may be removed or re-ordered in this list to affect the boot order. If
+the variable is empty, the default ordering is used, based on the priority of
+bootdevs and their sequence numbers.
+
+
+bootmeths
+~~~~~~~~~
+
+This environment variable can be used to control the list of bootmeths used and
+their ordering for example::
+
+   setenv bootmeths "syslinux efi"
+
+Entries may be removed or re-ordered in this list to affect the order the
+bootmeths are tried on each bootdev. If the variable is empty, the default
+ordering is used, based on the bootmeth sequence numbers, which can be
+controlled by aliases.
+
+The :ref:`usage/cmd/bootmeth:bootmeth command` (`bootmeth order`) operates in
+the same way as setting this variable.
+
+
+Bootdev uclass
+--------------
+
+The bootdev uclass provides an simple API call to obtain a bootflows from a
+device::
+
+   int bootdev_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
+                            struct bootflow *bflow);
+
+This takes a iterator which indicates the bootdev, partition and bootmeth to
+use. It returns a bootflow. This is the core of the bootdev implementation. The
+bootdev drivers that implement this differ depending on the media they are
+reading from, but each is responsible for returning a valid bootflow if
+available.
+
+A helper called `bootdev_find_in_blk()` makes it fairly easy to implement this
+function for each media device uclass, in a few lines of code.
+
+
+Bootdev drivers
+---------------
+
+A bootdev driver is typically fairly simple. Here is one for mmc::
+
+    static int mmc_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
+                    struct bootflow *bflow)
+    {
+        struct udevice *mmc_dev = dev_get_parent(dev);
+        struct udevice *blk;
+        int ret;
+
+        ret = mmc_get_blk(mmc_dev, &blk);
+        /*
+         * If there is no media, indicate that no more partitions should be
+         * checked
+         */
+        if (ret == -EOPNOTSUPP)
+            ret = -ESHUTDOWN;
+        if (ret)
+            return log_msg_ret("blk", ret);
+        assert(blk);
+        ret = bootdev_find_in_blk(dev, blk, iter, bflow);
+        if (ret)
+            return log_msg_ret("find", ret);
+
+        return 0;
+    }
+
+    static int mmc_bootdev_bind(struct udevice *dev)
+    {
+        struct bootdev_uc_plat *ucp = dev_get_uclass_plat(dev);
+
+        ucp->prio = BOOTDEVP_0_INTERNAL_FAST;
+
+        return 0;
+    }
+
+    struct bootdev_ops mmc_bootdev_ops = {
+        .get_bootflow    = mmc_get_bootflow,
+    };
+
+    static const struct udevice_id mmc_bootdev_ids[] = {
+        { .compatible = "u-boot,bootdev-mmc" },
+        { }
+    };
+
+    U_BOOT_DRIVER(mmc_bootdev) = {
+        .name        = "mmc_bootdev",
+        .id        = UCLASS_BOOTDEV,
+        .ops        = &mmc_bootdev_ops,
+        .bind        = mmc_bootdev_bind,
+        .of_match    = mmc_bootdev_ids,
+    };
+
+The implementation of the `get_bootflow()` method is simply to obtain the
+block device and call a bootdev helper function to do the rest. The
+implementation of `bootdev_find_in_blk()` checks the partition table, and
+attempts to read a file from a filesystem on the partition number given by the
+`@iter->part` parameter.
+
+Each bootdev has a priority, which indicates the order in which it is used.
+Faster bootdevs are used first, since they are more likely to be able to boot
+the device quickly.
+
+
+Device hierarchy
+----------------
+
+A bootdev device is a child of the media device. In this example, you can see
+that the bootdev is a sibling of the block device and both are children of
+media device::
+
+    mmc           0  [ + ]   bcm2835-sdhost        |   |-- mmc@7e202000
+    blk           0  [ + ]   mmc_blk               |   |   |-- mmc@7e202000.blk
+    bootdev       0  [   ]   mmc_bootdev           |   |   `-- mmc@7e202000.bootdev
+    mmc           1  [ + ]   sdhci-bcm2835         |   |-- sdhci@7e300000
+    blk           1  [   ]   mmc_blk               |   |   |-- sdhci@7e300000.blk
+    bootdev       1  [   ]   mmc_bootdev           |   |   `-- sdhci@7e300000.bootdev
+
+The bootdev device is typically created automatically in the media uclass'
+`post_bind()` method by calling `bootdev_setup_for_dev()`. The code typically
+something like this::
+
+    ret = bootdev_setup_for_dev(dev, "eth_bootdev");
+    if (ret)
+        return log_msg_ret("bootdev", ret);
+
+Here, `eth_bootdev` is the name of the Ethernet bootdev driver and `dev`
+is the ethernet device. This function is safe to call even if standard boot is
+not enabled, since it does nothing in that case. It can be added to all uclasses
+which implement suitable media.
+
+
+The bootstd device
+------------------
+
+Standard boot requires a single instance of the bootstd device to make things
+work. This includes global information about the state of standard boot. See
+`struct bootstd_priv` for this structure, accessed with `bootstd_get_priv()`.
+
+Within the devicetree, if you add bootmeth devices or a system bootdev, they
+should be children of the bootstd device. See `arch/sandbox/dts/test.dts` for
+an example of this.
+
+
+The system bootdev
+------------------
+
+Some bootmeths don't operate on individual bootdevs, but on the whole system.
+For example, the EFI boot manager does its own device scanning and does not
+make use of the bootdev devices. Such bootmeths can make use of the system
+bootdev, typically considered last, after everything else has been tried.
+
+
+.. _`Automatic Devices`:
+
+Automatic devices
+-----------------
+
+It is possible to define all the required devices in the devicetree manually,
+but it is not necessary. The bootstd uclass includes a `dm_scan_other()`
+function which creates the bootstd device if not found. If no bootmeth devices
+are found at all, it creates one for each available bootmeth driver as well as a
+system bootdev.
+
+If your devicetree has any bootmeth device it must have all of them that you
+want to use, as well as the system bootdev if needed, since no bootmeth devices
+will be created automatically in that case.
+
+
+Using devicetree
+----------------
+
+If a bootdev is complicated or needs configuration information, it can be
+added to the devicetree as a child of the media device. For example, imagine a
+bootdev which reads a bootflow from SPI flash. The devicetree fragment might
+look like this::
+
+    spi@0 {
+        flash@0 {
+            reg = <0>;
+            compatible = "spansion,m25p16", "jedec,spi-nor";
+            spi-max-frequency = <40000000>;
+
+            bootdev {
+                compatible = "u-boot,sf-bootdev";
+                offset = <0x2000>;
+                size = <0x1000>;
+            };
+        };
+    };
+
+The `sf-bootdev` driver can implement a way to read from the SPI flash, using
+the offset and size provided, and return that bootflow file back to the caller.
+When distro boot wants to read the kernel it calls disto_getfile() which must
+provide a way to read from the SPI flash. See `distro_boot()` at distro_boot_
+for more details.
+
+Of course this is all internal to U-Boot. All the distro sees is another way
+to boot.
+
+
+Configuration
+-------------
+
+Standard boot is enabled with `CONFIG_BOOTSTD`. Each bootmeth has its own CONFIG
+option also. For example, `CONFIG_BOOTMETH_DISTRO` enables support for distro
+boot from a disk.
+
+
+Available bootmeth drivers
+--------------------------
+
+Bootmeth drivers are provided for:
+
+   - distro boot from a disk (syslinux)
+   - distro boot from a network (PXE)
+   - EFI boot using bootefi
+   - EFI boot using boot manager
+
+
+Command interface
+-----------------
+
+Three commands are available:
+
+`bootdev`
+    Allows listing of available bootdevs, selecting a particular one and
+    getting information about it. See :doc:`../usage/cmd/bootdev`
+
+`bootflow`
+    Allows scanning one or more bootdevs for bootflows, listing available
+    bootflows, selecting one, obtaining information about it and booting it.
+    See :doc:`../usage/cmd/bootflow`
+
+`bootmeth`
+    Allow listing of available bootmethds and setting the order in which they
+    are tried. See :doc:`../usage/cmd/bootmeth`
+
+.. _BootflowStates:
+
+Bootflow states
+---------------
+
+Here is a list of states that a bootflow can be in:
+
+=======  =======================================================================
+State    Meaning
+=======  =======================================================================
+base     Starting-out state, indicates that no media/partition was found. For an
+         SD card socket it may indicate that the card is not inserted.
+media    Media was found (e.g. SD card is inserted) but no partition information
+         was found. It might lack a partition table or have a read error.
+part     Partition was found but a filesystem could not be read. This could be
+         because the partition does not hold a filesystem or the filesystem is
+         very corrupted.
+fs       Filesystem was found but the file could not be read. It could be
+         missing or in the wrong subdirectory.
+file     File was found and its size detected, but it could not be read. This
+         could indicate filesystem corruption.
+ready    File was loaded and is ready for use. In this state the bootflow is
+         ready to be booted.
+=======  =======================================================================
+
+
+Theory of operation
+-------------------
+
+This describes how standard boot progresses through to booting an operating
+system.
+
+To start. all the necessary devices must be bound, including bootstd, which
+provides the top-level `struct bootstd_priv` containing optional configuration
+information. The bootstd device is also holds the various lists used while
+scanning. This step is normally handled automatically by driver model, as
+described in `Automatic Devices`_.
+
+Bootdevs are also required, to provide access to the media to use. These are not
+useful by themselves: bootmeths are needed to provide the means of scanning
+those bootdevs. So, all up, we need a single bootstd device, one or more bootdev
+devices and one or more bootmeth devices.
+
+Once these are ready, typically a `bootflow scan` command is issued. This kicks
+of the iteration process, which involves looking through the bootdevs and their
+partitions one by one to find bootflows.
+
+Iteration is kicked off using `bootflow_scan_first()`, which calls
+`bootflow_scan_bootdev()`.
+
+The iterator is set up with `bootflow_iter_init()`. This simply creates an
+empty one with the given flags. Flags are used to control whether each
+iteration is displayed, whether to return iterations even if they did not result
+in a valid bootflow, whether to iterate through just a single bootdev, etc.
+
+Then the ordering of bootdevs is determined, by `bootdev_setup_iter_order()`. By
+default, the bootdevs are used in the order specified by the `boot_targets`
+environment variable (e.g. "mmc2 mmc0 usb"). If that is missing then their
+sequence order is used, as determined by the `/aliases` node, or failing that
+their order in the devicetree. For BOOTSTD_FULL, if there is a `bootdev-order`
+property in the bootstd node, then this is used as a final fallback. In any
+case, the iterator ends up with a `dev_order` array containing the bootdevs that
+are going to be used, with `num_devs` set to the number of bootdevs and
+`cur_dev` starting at 0.
+
+Next, the ordering of bootdevs is determined, by `bootmeth_setup_iter_order()`.
+By default the ordering is again by sequence number, i.e. the `/aliases` node,
+or failing that the order in the devicetree. But the `bootmeth order` command
+or `bootmeths` environment variable can be used to set up an ordering. If that
+has been done, the ordering is in `struct bootstd_priv`, so that ordering is
+simply copied into the iterator. Either way, the `method_order` array it set up,
+along with `num_methods`. Then `cur_method` is set to 0.
+
+At this point the iterator is ready to use, with the first bootdev and bootmeth
+selected. All the other fields are 0. This means that the current partition is
+0, which is taken to mean the whole device, since partition numbers start at 1.
+It also means that `max_part` is 0, i.e. the maximum partition number we know
+about is 0, meaning that, as far as we know, there is no partition table on this
+bootdev.
+
+With the iterator ready, `bootflow_scan_bootdev()` checks whether the current
+settings produce a valid bootflow. This is handled by `bootflow_check()`, which
+either returns 0 (if it got something) or an error if not (more on that later).
+If the `BOOTFLOWF_ALL` iterator flag is set, even errors are returned as
+incomplete bootflows, but normally an error results in moving onto the next
+iteration.
+
+The `bootflow_scan_next()` function handles moving onto the next iteration and
+checking it. In fact it sits in a loop doing that repeatedly until it finds
+something it wants to return.
+
+The actual 'moving on' part is implemented in `iter_incr()`. This is a very
+simple function. It increments the first counter. If that hits its maximum, it
+sets it to zero and increments the second counter. You can think of all the
+counters together as a number with three digits which increment in order, with
+the least-sigificant digit on the right, counting like this:
+
+   ========    =======    =======
+   bootdev     part       method
+   ========    =======    =======
+   0           0          0
+   0           0          1
+   0           0          2
+   0           1          0
+   0           1          1
+   0           1          1
+   1           0          0
+   1           0          1
+   ========    =======    =======
+
+The maximum value for `method` is `num_methods - 1` so when it exceeds that, it
+goes back to 0 and the next `part` is considered. The maximum value for that is
+`max_part`, which is initially zero for all bootdevs. If we find a partition
+table on that bootdev, `max_part` can be updated during the iteration to a
+higher value - see `bootdev_find_in_blk()` for that, described later. If that
+exceeds its maximum, then the next bootdev is used. In this way, iter_incr()
+works its way through all possibilities, moving forward one each time it is
+called.
+
+There is no expectation that iteration will actually finish. Quite often a
+valid bootflow is found early on. With `bootflow scan -b`, that causes the
+bootflow to be immediately booted. Assuming it is successful, the iteration never
+completes.
+
+Also note that the iterator hold the **current** combination being considered.
+So when `iter_incr()` is called, it increments to the next one and returns it,
+the new **current** combination.
+
+Note also the `err` field in `struct bootflow_iter`. This is normally 0 and has
+thus has no effect on `iter_inc()`. But if it is non-zero, signalling an error,
+it indicates to the iterator what it should do when called. It can force moving
+to the next partition, or bootdev, for example. The special values
+`BF_NO_MORE_PARTS` and `BF_NO_MORE_DEVICES` handle this. When `iter_incr` sees
+`BF_NO_MORE_PARTS` it knows that it should immediately move to the next bootdev.
+When it sees `BF_NO_MORE_DEVICES` it knows that there is nothing more it can do
+so it should immediately return. The caller of `iter_incr()` is responsible for
+updating the `err` field, based on the return value it sees.
+
+The above describes the iteration process at a high level. It is basically a
+very simple increment function with a checker called `bootflow_check()` that
+checks the result of each iteration generated, to determine whether it can
+produce a bootflow.
+
+So what happens inside of `bootflow_check()`? It simply calls the uclass
+method `bootdev_get_bootflow()` to ask the bootdev to return a bootflow. It
+passes the iterator to the bootdev method, so that function knows what we are
+talking about. At first, the bootflow is set up in the state `BOOTFLOWST_BASE`,
+with just the `method` and `dev` intiialised. But the bootdev may fill in more,
+e.g. updating the state, depending on what it finds.
+
+Based on what the bootdev responds with, `bootflow_check()` either
+returns a valid bootflow, or a partial one with an error. A partial bootflow
+is one that has some fields set up, but did not reach the `BOOTFLOWST_READY`
+state. As noted before, if the `BOOTFLOWF_ALL` iterator flag is set, then all
+bootflows are returned, even partial ones. This can help with debugging.
+
+So at this point you can see that total control over whether a bootflow can
+be generated from a particular iteration, or not, rests with the bootdev.
+Each one can adopt its own approach.
+
+Going down a level, what does the bootdev do in its `get_bootflow()` method?
+Let us consider the MMC bootdev. In that case the call to
+`bootdev_get_bootflow()` ends up in `mmc_get_bootflow()`. It locates the parent
+device of the bootdev, i.e. the `UCLASS_MMC` device itself, then finds the block
+device associated with it. It then calls the helper function
+`bootdev_find_in_blk()` to do all the work. This is common with just about any
+bootdev that is based on a media device.
+
+The `bootdev_find_in_blk()` helper is implemented in the bootdev uclass. It
+names the bootflow and copies the partition number in from the iterator. Then it
+calls the bootmeth device to check if it can support this device. This is
+important since some bootmeths only work with network devices, for example. If
+that check fails, it stops.
+
+Assuming the bootmeth is happy, or at least indicates that it is willing to try
+(by returning 0 from its `check()` method), the next step is to try the
+partition. If that works it tries to detect a file system. If that works then it
+calls the bootmeth device once more, this time to read the bootflow.
+
+Note: At present a filesystem is needed for the bootmeth to be called on block
+devices, simply because we don't have any examples where this is not the case.
+This feature can be added as needed.
+
+If we take the example of the `bootmeth_distro` driver, this call ends up at
+`distro_read_bootflow()`. It has the filesystem ready, so tries various
+filenames to try to find the `extlinux.conf` file, reading it if possible. If
+all goes well the bootflow ends up in the `BOOTFLOWST_READY` state.
+
+At this point, we fall back from the bootmeth driver, to
+`bootdev_find_in_blk()`, then back to `mmc_get_bootflow()`, then to
+`bootdev_get_bootflow()`, then to `bootflow_check()` and finally to its caller,
+either `bootflow_scan_bootdev()` or `bootflow_scan_next()`. In either case,
+the bootflow is returned as the result of this iteration, assuming it made it to
+the  `BOOTFLOWST_READY` state.
+
+That is the basic operation of scanning for bootflows. The process of booting a
+bootflow is handled by the bootmeth driver for that bootflow. In the case of
+distro boot, this parses and processes the `extlinux.conf` file that was read.
+See `distro_boot()` for how that works. The processing may involve reading
+additional files, which is handled by the `read_file()` method, which is
+`distro_read_file()` in this case. All bootmethds should support reading files,
+since the bootflow is typically only the basic instructions and does not include
+the operating system itself, ramdisk, device tree, etc.
+
+The vast majority of the bootstd code is concerned with iterating through
+partitions on bootdevs and using bootmethds to find bootflows.
+
+How about bootdevs which are not block devices? They are handled by the same
+methods as above, but with a different implementation. For example, the bootmeth
+for PXE boot (over a network) uses `tftp` to read files rather than `fs_read()`.
+But other than that it is very similar.
+
+
+Tests
+-----
+
+Tests are located in `test/boot` and cover the core functionality as well as
+the commands. All tests use sandbox so can be run on a standard Linux computer
+and in U-Boot's CI.
+
+For testing, a DOS-formatted disk image is used with a single FAT partition on
+it. This is created in `setup_bootflow_image()`, with a canned one from the
+source tree used if it cannot be created (e.g. in CI).
+
+
+Bootflow internals
+------------------
+
+The bootstd device holds a linked list of scanned bootflows as well as the
+currently selected bootdev and bootflow (for use by commands). This is in
+`struct bootstd_priv`.
+
+Each bootdev device has its own `struct bootdev_uc_plat` which holds a
+list of scanned bootflows just for that device.
+
+The bootflow itself is documented in bootflow_h_. It includes various bits of
+information about the bootflow and a buffer to hold the file.
+
+
+Future
+------
+
+Apart from the to-do items below, different types of bootflow files may be
+implemented in future, e.g. Chromium OS support which is currently only
+available as a script in chromebook_coral.
+
+
+To do
+-----
+
+Some things that need to be done to completely replace the distro-boot scripts:
+
+- add bootdev drivers for dhcp, sata, scsi, ide, virtio
+- PXE boot for EFI
+- support for loading U-Boot scripts
+
+Other ideas:
+
+- `bootflow prep` to load everything preparing for boot, so that `bootflow boot`
+  can just do the boot.
+- automatically load kernel, FDT, etc. to suitable addresses so the board does
+  not need to specify things like `pxefile_addr_r`
+
+
+.. _disto_boodcmd: https://github.com/u-boot/u-boot/blob/master/include/config_distro_bootcmd.h
+.. _BootLoaderSpec: http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/
+.. _distro_boot: https://github.com/u-boot/u-boot/blob/master/boot/distro.c
+.. _bootflow_h: https://github.com/u-boot/u-boot/blob/master/include/bootflow.h