linux-kernel - Re: [PATCH 1/2] spi: new SPI bus lock/unlock functions

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Date:	Mon, 21 Sep 2009 09:36:25 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Li Yi <yi.li@...log.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	spi-devel-general@...ts.sourceforge.net,
	dbrownell@...rs.sourceforge.net, linux-kernel@...r.kernel.org,
	cooloney@...nel.org
Subject: Re: [PATCH 1/2] spi: new SPI bus lock/unlock functions

On Mon, Sep 21, 2009 at 02:33, Li Yi wrote:
> On Fri, 2009-09-18 at 14:29 -0700, Andrew Morton wrote:
>> On Thu, 17 Sep 2009 18:03:16 -0400
>> Mike Frysinger <vapier@...too.org> wrote:
>>
>> > From: Yi Li <yi.li@...log.com>
>> >
>> > For some MMC cards over SPI bus, it needs to lock the SPI bus for its own
>> > use.  The SPI transfer must not be interrupted by other SPI devices that
>> > share the SPI bus with SPI MMC card.
>> >
>> > This patch introduces 2 APIs for SPI bus locking operation.
>> >
>> > Signed-off-by: Yi Li <yi.li@...log.com>
>> > Signed-off-by: Bryan Wu <cooloney@...nel.org>
>> > Signed-off-by: Mike Frysinger <vapier@...too.org>
>> > ---
>> > Andrew: we've posted these in the past with no response.  could you pick
>> >         them up please ?
>> >
>> >  drivers/spi/spi.c       |   48 +++++++++++++++++++++++++++++++++++++++++++++++
>> >  include/linux/spi/spi.h |    7 ++++++
>> >  2 files changed, 55 insertions(+), 0 deletions(-)
>> >
>> > diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
>> > index 70845cc..b82b8ad 100644
>> > --- a/drivers/spi/spi.c
>> > +++ b/drivers/spi/spi.c
>> > @@ -653,6 +653,54 @@ static void spi_complete(void *arg)
>> >  }
>> >
>> >  /**
>> > + * spi_lock_bus - lock SPI bus for exclusive access
>> > + * @spi: device which want to lock the bus
>> > + * Context: any
>> > + *
>> > + * Once the caller owns exclusive access to the SPI bus,
>> > + * only messages for this device will be transferred.
>> > + * Messages for other devices are queued but not transferred until
>> > + * the bus owner unlock the bus.
>> > + *
>> > + * The caller may call spi_lock_bus() before spi_sync() or spi_async().
>> > + * So this call may be used in irq and other contexts which can't sleep,
>> > + * as well as from task contexts which can sleep.
>>
>> Hence spi_lock_bus() basically has to use a spinning lock?
>>
>> So code which has called spi_lock_bus() cannot sleep until it calls
>> spi_unlock_bus()?
>>
>> That's worth mentioning in the description.
>>
> Code called spi_lock_bus() _can_ sleep. This is mentioned in the
> comments.

should add might_sleep() to the common spi_lock_bus() function then.
-mike
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