linux-kernel - Re: [PATCH] spi: Unlock a spinlock before calling into the controller driver.

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Message-ID: <CACRpkdbuiHmZP7HKuoaw1O0xyZJEDv6ja1Q=YqOtBViRYBufZQ@mail.gmail.com>
Date:	Tue, 26 Jun 2012 02:56:17 +0800
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Olof Johansson <olof@...om.net>, Bryan Freed <bfreed@...omium.org>,
	spi-devel-general@...ts.sourceforge.net, grant.likely@...retlab.ca,
	linux-kernel@...r.kernel.org,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Subject: Re: [PATCH] spi: Unlock a spinlock before calling into the controller driver.

On Tue, Jun 26, 2012 at 1:07 AM, Doug Anderson <dianders@...omium.org> wrote:

> Specifically there should be only one instance of spi_pump_messages()
> running at a time per master.  That's because it's a kthread work
> function.  ...so we can't possibly get a prepare in the middle of the
> unprepare when prepare is called because the only caller to
> prepare/unprepare is spi_pump_messages().

Yes that's how the message pump is designed.

> I can't comment on whether it's better to do something like add a
> workqueue (which might be more obvious / less fragile) or just to add
> a comment.  I will let others comment on that.  :)

The message pump initially used a workqueue, but was converted
to a kthread because we needed to push the queue to run as
realtime for some important low-latency workloads across
SPI. The code is basically a tweaked workqueue if you dive down
in the implementation.

Yours,
Linus Walleij
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