linux-kernel - Re: [PATCH] serdev: Restore serdev_device_write

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Date:   Tue, 2 May 2017 15:18:54 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Johan Hovold <johan@...nel.org>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Sebastian Reichel <sre@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] serdev: Restore serdev_device_write_buf for atomic
 context

On Tue, May 02, 2017 at 07:41:34AM -0500, Rob Herring wrote:
> On Tue, May 2, 2017 at 4:06 AM, Johan Hovold <johan@...nel.org> wrote:
> > On Fri, Apr 28, 2017 at 01:47:21PM +0200, Stefan Wahren wrote:
> >> Starting with commit 6fe729c4bdae ("serdev: Add serdev_device_write
> >> subroutine") the function serdev_device_write_buf cannot be used in
> >> atomic context anymore (mutex_lock is sleeping). So restore the old
> >> behavior.
> >
> > Yeah, preventing use in atomic context seems unnecessary, although any
> > clients writing must now deal with serialisation themselves (as before,
> > and as they should).
> 
> We could just remove the mutex for serdev_device_write and always make
> the client responsible for serialization.

That sounds reasonable.

> > Calling wait_for_completion in the non-blocking case was also needlessly
> > inefficient.
> 
> It won't be called because count should be 0.

That's not guaranteed; count would be nonzero whenever the tty
driver does not accept the full buffer and then we'd currently end up
calling wait_for_completion_timeout() with a zero-timeout instead of
just returning immediately.

Johan