Re: [Linux-nvdimm] [PATCH v2 18/20] libnd: infrastructure for btt devices

[Toshi Kani <toshi.kani@hp.com>]

On Tue, 2015-04-28 at 14:25 -0400, Dan Williams wrote:
> Block devices from an nd bus, in addition to accepting "struct bio"
> based requests, also have the capability to perform byte-aligned
> accesses.  By default only the bio/block interface is used.  However, if
> another driver can make effective use of the byte-aligned capability it
> can claim/disable the block interface and use the byte-aligned "nd_io"
> interface.
> 
> The BTT driver is the intended first consumer of this mechanism to allow
> layering atomic sector update guarantees on top of nd_io capable
> nd-bus-block-devices.
 :
> +static int nd_btt_autodetect(struct nd_bus *nd_bus, struct nd_io *ndio,
> +		struct block_device *bdev)
> +{
> +	char name[BDEVNAME_SIZE];
> +	struct nd_btt *nd_btt;
> +	struct btt_sb *btt_sb;
> +	u64 offset, checksum;
> +	u32 lbasize;
> +	u8 *uuid;
> +	int rc;
> +
> +	btt_sb = kzalloc(sizeof(*btt_sb), GFP_KERNEL);
> +	if (!btt_sb)
> +		return -ENODEV;
> +
> +	offset = nd_partition_offset(bdev);
> +	rc = ndio->rw_bytes(ndio, btt_sb, offset + SZ_4K, sizeof(*btt_sb), READ);
> +	if (rc)
> +		goto out_free_sb;
> +
> +	if (get_capacity(bdev->bd_disk) < SZ_16M / 512)
> +		goto out_free_sb;
> +
> +	if (memcmp(btt_sb->signature, BTT_SIG, BTT_SIG_LEN) != 0)
> +		goto out_free_sb;
> +
> +	checksum = le64_to_cpu(btt_sb->checksum);
> +	btt_sb->checksum = 0;
> +	if (checksum != nd_btt_sb_checksum(btt_sb))
> +		goto out_free_sb;
> +	btt_sb->checksum = cpu_to_le64(checksum);
> +
> +	uuid = kmemdup(btt_sb->uuid, 16, GFP_KERNEL);
> +	if (!uuid)
> +		goto out_free_sb;
> +
> +	lbasize = le32_to_cpu(btt_sb->external_lbasize);
> +	nd_btt = __nd_btt_create(nd_bus, lbasize, uuid);

When BTT is first set up, user binds a seed "btt0" to a block device,
such as /dev/pmem0.  It then creates /dev/nd0 bound to /dev/pmem0.

After a reboot, nd_btt_autodetect() detects the BTT setup and creates a
new "btt1" since it is called after a seed "btt0" is created.
Therefore, it creates /dev/nd1 bound to /dev/pmem0 this time.

Is this how it is intended to work, i.e. "btt0" as the default seed btt?
While user should not rely on the name of /dev/nd%d, I thought this
device name change was confusing...

Thanks,
-Toshi