linux-kernel - Re: [PATCH v14 00/11] mux controller abstraction and iio/i2c muxes

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Date:   Tue, 25 Apr 2017 18:32:57 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Jonathan Cameron <jic23@...nel.org>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Jonathan Corbet <corbet@....net>, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Ian King <colin.king@...onical.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        kernel@...gutronix.de
Subject: Re: [PATCH v14 00/11] mux controller abstraction and iio/i2c muxes

On Tue, 2017-04-25 at 16:55 +0200, Peter Rosin wrote:
> On 2017-04-25 16:16, Peter Rosin wrote:
> > On 2017-04-24 16:59, Philipp Zabel wrote:
> >> On Mon, 2017-04-24 at 16:36 +0200, Peter Rosin wrote:
> >> [...]
> >>>> How about an atomic use_count on the mux_control, a bool shared that is
> >>>> only set by the first consumer, and controls whether selecting locks?
> >>>
> >>> That has the drawback that it is hard to restore the mux-control in a safe
> >>> way so that exclusive consumers are allowed after the last shared consumer
> >>> puts the mux away.
> >>
> >> True.
> >>
> >>> Agreed, it's a corner case, but I had this very similar
> >>> patch going through the compiler when I got this mail. Does it work as well
> >>> as what you suggested?
> >>
> >> Yes, this patch works just as well.
> > 
> > Right, as expected :-) However, I don't like it much. It divides the mux
> > consumers into two camps in a way that makes it difficult to select which
> > camp a consumer should be in.
> > 
> > E.g. consider the iio-mux. The current implementation only supports quick
> > accesses that fit the mux_control_get_shared case. But if that mux in the
> > future needs to grow continuous buffered accesses, I think there will be
> > pressure to switch it over to the exclusive mode. Because that is a lot
> > closer to what you are doing with the video-mux. And then what? It will be
> > impossible to predict if the end user is going to use buffered accesses or
> > not...
> > 
> > So, I think the best approach is to skip the distinction between shared
> > and exclusive consumers and instead implement the locking with an ordinary
> > semaphore (instead of the old rwsem or the current mutex). Semaphores don't
> > have the property that the same task should down/up them (mutexes require
> > that for lock/unlock, and is also the reason for the lockdep complaint) and
> > thus fits better for long-time use such as yours or the above iio-mux with
> > buffered accesses. It should also hopefully be cheaper that an rwsem, and
> > not have any downgrade_write calls thus possibly keeping Greg sufficiently
> > happy...

No idea whether this will placate Greg, but it does work for the
video-mux case.
The documentation for mux_control_(try_)select should mention that these
calls will hold the mux lock until deselect is called, and the
documentation for mux_control_select should probably mention that it
will block until the lock is released.

> > Sure, consumers can still dig themselves into a hole by not calling deselect
> > as they should, but at least I think it can be made to work w/o dividing the
> > consumers...
> 
> Like this (only compile-tested). Philipp, it should work the same as with
> the rwsem in v13 and earlier. At least for your case...

regards
Philipp