[dm-devel] [PATCH V5 11/12] block: add poll_capable method to support bio-based IO polling

[Christoph Hellwig]

On Thu, Apr 01, 2021 at 10:19:26AM +0800, Ming Lei wrote:
> From: Jeffle Xu <jefflexu at linux.alibaba.com>
> 
> This method can be used to check if bio-based device supports IO polling
> or not. For mq devices, checking for hw queue in polling mode is
> adequate, while the sanity check shall be implementation specific for
> bio-based devices. For example, dm device needs to check if all
> underlying devices are capable of IO polling.
> 
> Though bio-based device may have done the sanity check during the
> device initialization phase, cacheing the result of this sanity check
> (such as by cacheing in the queue_flags) may not work. Because for dm
> devices, users could change the state of the underlying devices through
> '/sys/block/<dev>/io_poll', bypassing the dm device above. In this case,
> the cached result of the very beginning sanity check could be
> out-of-date. Thus the sanity check needs to be done every time 'io_poll'
> is to be modified.

I really don't think thi should be a method, and I really do dislike
how we have all this "if (is_mq)" junk.  Why can't we have a flag on
the gendisk that signals if the device can support polling that
is autoamtically set for blk-mq and as-needed by bio based drivers?
And please move everything that significantly hanges things for the
mq based path to separate prep patches early in th series.