[dm-devel] How can I get my sector size (via queue_limits?) from a dm_target?

[Mike Snitzer]

On Tue, Jan 19 2016 at 10:32pm -0500,
Eric Wheeler <dm-devel at lists.ewheeler.net> wrote:

> Hello all,
> 
> dm_target->len gives me sector count, but I need to know the default 
> sector size discovered by the blk stacking code without assuming 512b.
> 
> Now that I have the dm .iterate_devices callback defined and my 
> queue_limits are sane by default, how can I get a pointer to the 
> queue_limits structure for my device given a dm_target structure?
> 
> It looks like I should call dm_get_md_queue() after getting the md from 
> dm_table_get_md(target->table), but dm_get_md_queue is not an exported 
> symbol.
> 
> Also, is that value correct within the .ctr, or only after the .ctr 
> returns?  If after, how can I hook in after limits are assigned but before 
> .map gets called?  Otherwise stated, I would like to know the stacked 
> queue_limits corrected for lower layers within the .ctr function.

You cannot know it from .ctr because the device is actively being
constructed.  The .io_hints hook allows you access to stacked
queue_limits -- see dm_calculate_queue_limits() which calls ->io_hints
_after_ combining all underlying queue_limits.

See dm-thin.c's pool_io_hints() and thin_io_hints() for examples of how
the .io_hints hook is used for DM-thinp.

Mike