[dm-devel] [PATCH] block: introduce QUEUE_FLAG_POLL_CAP flag

[Ming Lei]

On Fri, Apr 16, 2021 at 04:00:37PM +0800, Jeffle Xu wrote:
> Hi,
> How about this patch to remove the extra poll_capable() method?
> 
> And the following 'dm: support IO polling for bio-based dm device' needs
> following change.
> 
> ```
> +       /*
> +        * Check for request-based device is remained to
> +        * dm_mq_init_request_queue()->blk_mq_init_allocated_queue().
> +        * For bio-based device, only set QUEUE_FLAG_POLL when all underlying
> +        * devices supporting polling.
> +        */
> +       if (__table_type_bio_based(t->type)) {
> +               if (dm_table_supports_poll(t)) {
> +                       blk_queue_flag_set(QUEUE_FLAG_POLL_CAP, q);
> +                       blk_queue_flag_set(QUEUE_FLAG_POLL, q);
> +               }
> +               else {
> +                       blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
> +                       blk_queue_flag_clear(QUEUE_FLAG_POLL_CAP, q);
> +               }
> +       }
> ```

Frankly speaking, I don't see any value of using QUEUE_FLAG_POLL_CAP for
DM, and the result is basically subset of treating DM as always being capable
of polling.

Also underlying queue change(either limits or flag) won't be propagated
to DM/MD automatically. Strictly speaking it doesn't matter if all underlying
queues are capable of supporting polling at the exact time of 'write sysfs/poll',
cause any of them may change in future.

So why not start with the simplest approach(always capable of polling)
which does meet normal bio based polling requirement?



Thanks,
Ming