[RFC/PATCH v2] ubi: Add ubiblock read-write driver

[Mike Frysinger]

On Wednesday 12 December 2012 07:21:52 Ezequiel Garcia wrote:
> Block device emulation on top of ubi volumes with read/write support.
> Block devices are created upon user request through the
> 'vol' module parameter.
> 
> For instance,
> 
>   $ modprobe ubiblock vol=/dev/ubi0_0
>   $ modprobe ubiblock vol=0,rootfs

i played around with ubiblk before finding this newer version.  one thing i 
think this is missing that ubiblk had is an ioctl interface for adding new 
block volumes on the fly.  you can attach ubi volumes at runtime, but the only 
way to attach ubiblocks is by loading/unloading the module, or by rebooting 
and tweaking the command line.

imo, that support needs to be re-added.  it'd be great if we could do it via 
the existing /dev/ubi_ctrl knob, but maybe that'll only work if ubi+ubiblock 
are built into the kernel, or if ubiblock is merged with ubi ?

> Read/write access is expected to work fairly well because the
> request queue at block elevator orders block transfers to achieve
> space-locality.
> In other words, it's expected that reads and writes gets ordered
> to address the same LEB.

i wonder if the write support should be put behind a CONFIG option.  
personally, the write support is kind of neat and semi-useful for development, 
but i don't plan on shipping anything on that :).  i just want read-only 
support to load an ext2 fs on top of UBI.

> +static int ubiblock_open(struct block_device *bdev, fmode_t mode)
> +{
> +	struct ubiblock *dev = bdev->bd_disk->private_data;
> +	int ubi_mode = UBI_READONLY;
> +	int ret;
> +
> +	mutex_lock(&dev->vol_mutex);
> +	if (dev->refcnt > 0) {
> +		/*
> +		 * The volume is already opened,
> +		 * just increase the reference counter
> +		 */
> +		dev->refcnt++;
> +		mutex_unlock(&dev->vol_mutex);
> +		return 0;
> +	}
> +
> +	if (mode & FMODE_WRITE)
> +		ubi_mode = UBI_READWRITE;

hmm, you handle ro vs rw here ...

> +	ret = ubiblock_alloc_cache(&dev->read_cache, dev->leb_size);
> +	if (ret)
> +		goto out_free;
> +
> +	ret = ubiblock_alloc_cache(&dev->write_cache, dev->leb_size);
> +	if (ret)
> +		goto out_free_cache;

... but you always alloc a write cache even when it's mounted ro ?
-mike
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