Re: [PATCH 3/5] dm: linear: Add linear target

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hi,

On 28.08.25 17:05, Tobias Waldekranz wrote:
> This target allows you to map in a region of another device as a
> contiguous range of blocks of a dm device.
> 
> This is the basic building block used by LVM to stitch together
> logical volumes from one or more ranges of blocks from one or more
> physical disks.

I have yet to read through the patch series, but a thought coming to mind:

We have storage-by-alias driver, which creates new cdevs that 1:1 map
an existing cdev, but given that they are separate, they allow a different
fixed partitioning. This was originally useful when running under EFI to
make partitions known to barebox that are outside of a MBR and are not
described in the device tree. More recently, they provide the
barebox,bootsource driver which allows referencing in the DT the bootsource
in a generic manner.

I am wondering if this at some future point of time could be addressed
using the DM linear API. What comes to mind is that instead of only dmsetup,
DT nodes would also be able to create at least a linear dm target and
then we can drop most of code in the storage-by-alias driver.

Exciting times ahead!

Cheers,
Ahmad


> 
> Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
> ---
>  drivers/block/dm/Kconfig     |   7 ++
>  drivers/block/dm/Makefile    |   1 +
>  drivers/block/dm/dm-linear.c | 123 +++++++++++++++++++++++++++++++++++
>  3 files changed, 131 insertions(+)
>  create mode 100644 drivers/block/dm/dm-linear.c
> 
> diff --git a/drivers/block/dm/Kconfig b/drivers/block/dm/Kconfig
> index f64c69e03d..e763e530d3 100644
> --- a/drivers/block/dm/Kconfig
> +++ b/drivers/block/dm/Kconfig
> @@ -5,3 +5,10 @@ menuconfig DM_BLK
>  	  other data sources. Modeled after, and intended to be
>  	  compatible with, the Linux kernel's device mapper subsystem.
>  
> +config DM_BLK_LINEAR
> +        bool "Linear target"
> +	depends on DM_BLK
> +        help
> +	  Maps a contiguous region of an existing device into a dm
> +	  device.
> +
> diff --git a/drivers/block/dm/Makefile b/drivers/block/dm/Makefile
> index 9c045087c0..f52d19f9c4 100644
> --- a/drivers/block/dm/Makefile
> +++ b/drivers/block/dm/Makefile
> @@ -1,2 +1,3 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  obj-$(CONFIG_DM_BLK) += dm-core.o
> +obj-$(CONFIG_DM_BLK_LINEAR) += dm-linear.o
> diff --git a/drivers/block/dm/dm-linear.c b/drivers/block/dm/dm-linear.c
> new file mode 100644
> index 0000000000..d3415814cb
> --- /dev/null
> +++ b/drivers/block/dm/dm-linear.c
> @@ -0,0 +1,123 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +// SPDX-FileCopyrightText: © 2025 Tobias Waldekranz <tobias@waldekranz.com>, Wires
> +
> +#include <block.h>
> +#include <common.h>
> +#include <disks.h>
> +#include <fcntl.h>
> +
> +#include "dm-target.h"
> +
> +struct dm_linear {
> +	struct cdev *cdev;
> +	loff_t offset;
> +};
> +
> +static int dm_linear_read(struct dm_target *ti, void *buf,
> +			  sector_t block, blkcnt_t num_blocks)
> +{
> +	struct dm_linear *l = ti->private;
> +
> +	block <<= SECTOR_SHIFT;
> +	num_blocks <<= SECTOR_SHIFT;
> +
> +	if (cdev_read(l->cdev, buf, num_blocks, l->offset + block, 0) < num_blocks)
> +		return -EIO;
> +
> +	return 0;
> +}
> +
> +static int dm_linear_write(struct dm_target *ti, const void *buf,
> +			   sector_t block, blkcnt_t num_blocks)
> +{
> +	struct dm_linear *l = ti->private;
> +
> +	block <<= SECTOR_SHIFT;
> +	num_blocks <<= SECTOR_SHIFT;
> +
> +	if (cdev_write(l->cdev, buf, num_blocks, l->offset + block, 0) < num_blocks)
> +		return -EIO;
> +
> +	return 0;
> +}
> +
> +static int dm_linear_create(struct dm_target *ti, unsigned int argc, char **argv)
> +{
> +	struct dm_linear *l;
> +	int err;
> +
> +	if (argc != 2) {
> +		dm_target_err(ti, "Expected 2 arguments (\"<dev> <offset>\"), got %u\n",
> +			      argc);
> +		err = -EINVAL;
> +		goto err;
> +	}
> +
> +	l = xzalloc(sizeof(*l));
> +	ti->private = l;
> +
> +	if (kstrtoull(argv[1], 0, &l->offset)) {
> +		dm_target_err(ti, "Invalid offset: \"%s\"\n", argv[1]);
> +		err = -EINVAL;
> +		goto err_free;
> +	}
> +	l->offset <<= SECTOR_SHIFT;
> +
> +	l->cdev = cdev_open_by_path_name(argv[0], O_RDWR);
> +	if (!l->cdev) {
> +		dm_target_err(ti, "Cannot open device %s: %m\n", argv[0]);
> +		err = -ENODEV;
> +		goto err_free;
> +	}
> +
> +	l->cdev = cdev_readlink(l->cdev);
> +
> +	if ((ti->size << SECTOR_SHIFT) > (l->cdev->size - l->offset)) {
> +		dm_target_err(ti, "%s is too small to map %llu blocks at %llu, %llu available\n",
> +			      argv[0], ti->size, l->offset >> SECTOR_SHIFT,
> +			      (l->cdev->size - l->offset) >> SECTOR_SHIFT);
> +		err = -ENOSPC;
> +		goto err_close;
> +	}
> +
> +	return 0;
> +
> +err_close:
> +	cdev_close(l->cdev);
> +err_free:
> +	free(l);
> +err:
> +	return err;
> +}
> +
> +static int dm_linear_destroy(struct dm_target *ti)
> +{
> +	struct dm_linear *l = ti->private;
> +
> +	cdev_close(l->cdev);
> +	free(l);
> +	return 0;
> +}
> +
> +static char *dm_linear_asprint(struct dm_target *ti)
> +{
> +	struct dm_linear *l = ti->private;
> +
> +	return xasprintf("dev:%s offset:%llu",
> +			 cdev_name(l->cdev), l->offset >> SECTOR_SHIFT);
> +}
> +
> +static struct dm_target_ops dm_linear_ops = {
> +	.name = "linear",
> +	.asprint = dm_linear_asprint,
> +	.create = dm_linear_create,
> +	.destroy = dm_linear_destroy,
> +	.read = dm_linear_read,
> +	.write = dm_linear_write,
> +};
> +
> +static int __init dm_linear_init(void)
> +{
> +	return dm_target_register(&dm_linear_ops);
> +}
> +device_initcall(dm_linear_init);


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