linux-kernel - Re: [RFC PATCH 1/2] iio: core: implement iio_{claim|release}_direct

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Date:	Wed, 9 Mar 2016 12:06:56 -0800
From:	Alison Schofield <amsfield22@...il.com>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Lars-Peter Clausen <lars@...afoo.de>,
	outreachy-kernel@...glegroups.com, knaack.h@....de,
	pmeerw@...erw.net, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org, Michael.Hennerich@...log.com,
	gregkh@...uxfoundation.org, devel@...verdev.osuosl.org
Subject: Re: [RFC PATCH 1/2] iio: core: implement
 iio_{claim|release}_direct_mode()

On Sat, Mar 05, 2016 at 06:02:36PM +0000, Jonathan Cameron wrote:
> On 02/03/16 13:28, Lars-Peter Clausen wrote:
> > On 03/01/2016 08:02 PM, Alison Schofield wrote:
> >> It is often the case that the driver wants to be sure a device stays
> >> in direct mode while it is executing a task or series of tasks.  To
> >> accomplish this today, the driver performs this sequence: 1) take the
> >> device state lock, 2)verify it is not in a buffered mode, 3) execute
> >> some tasks, and 4) release that lock.
> >>
> >> This patch introduces a pair of helper functions that simplify these
> >> steps and make it more semantically expressive.
> >>
> >> iio_claim_direct_mode()
> >>         If the device is not in any buffered mode it is guaranteed
> >>         to stay that way until iio_release_direct_mode() is called.
> >>
> >> iio_release_direct_mode()
> >>         Release the claim. Device is no longer guaranteed to stay
> >>         in direct mode.
> >>
> >> Signed-off-by: Alison Schofield <amsfield22@...il.com>
> > 
> > Looks basically good.
> Agreed - nothing to add from me to what Lars has covered here.
> Nice to 'hide' the accesses to mlock as well as will cut out the desire
> to 'abuse it'.  Amusingly we only just 'fixed' the docs to to say this
> element of iio_dev was usable by drivers.  Once we have these new functions
> in use throughout the tree, we will want to flip that back again to internal
> only.
> 
> Jonathan
>
Thanks for the review (& Lars too)  

Thinking about your note about flipping the mlock field back to
INTERNAL (from DRIVER), this change, even when it's applied to
all relevant instances, doesn't get us all the way there.

While these claim/release functions will remove direct access to mlock
where a driver wants to hold direct mode, the drivers are grabbing
mlock for other reasons also.  (too many reasons/instances for me to
quickly understand or summarize)  

I'm willing to look at it further and comment if that's helpful.

alisons