Re: [PATCH 1/5] spi: spi_lock_bus and spi_unlock_bus

[Mike Frysinger <vapier.adi-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Tue, Feb 16, 2010 at 22:48, Grant Likely wrote:
> On Tue, Feb 16, 2010 at 5:07 PM, Mike Frysinger wrote:
>> On Tue, Feb 16, 2010 at 15:43, Grant Likely wrote:
>>> On Tue, Feb 16, 2010 at 12:57 PM, Ernst Schwab wrote:
>>>> From: Yi Li <yi.li@analog.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@analog.com>
>>>> Signed-off-by: Bryan Wu <cooloney@kernel.org>
>>>> Signed-off-by: Mike Frysinger <vapier@gentoo.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.
>>>> + *
>>>> + * It returns zero on success, else a negative error code.
>>>> + */
>>>> +int spi_lock_bus(struct spi_device *spi)
>>>> +{
>>>> +       if (spi->master->lock_bus)
>>>> +               return spi->master->lock_bus(spi);
>>>> +       else
>>>> +               return 0;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(spi_lock_bus);
>>>
>>> This series seems to try and solve the problem the hard way, and by
>>> creating a new locking scheme (and as history shows, new locking
>>> schemes are *alwasy* broken).
>>>
>>> Why is the locking getting pushed down to the bus driver level?  It
>>> seems to me that the whole thing could be handled with common code and
>>> a mutex in the spi_master structure.  spi_sync would be easy to handle
>>> by putting a mutex around the spi_message submission.  spi_async would
>>> be a little harder since it needs to be atomic, but that could also be
>>> handled with a flag protected by a spinlock.
>>>
>>> Basically, the idea is that existing drivers continue to use the API as-is
>>>
>>> Drivers that want to lock the bus for exclusive access must call
>>> spi_lock_bus() which should take the mutex and then sleep until all
>>> in-flight spi_messages are processed.  After that, anyone calling
>>> spi_async() will simply sleep until the locker unlocks the bus again.
>>>
>>> To handle spi_sync() would probably require a flag protected by a
>>> spinlock.  If the flag is set, then spi_sync() would simply fail.
>>>
>>> Finally, the locking driver would need locked versions of spi_sync()
>>> and spi_async() that sidestep the lock checks.  It would only be valid
>>> to call these versions when holding the SPI bus lock.
>>>
>>> There is no need to specify the spi_device in the lock request.  Since
>>> the lock is exclusive, it is known that the only driver calling the
>>> locked API version must already hold the lock.
>>
>> this is what i proposed last time, but we havent gotten around to
>> implementing it:
>>
>> there's nothing Blackfin-specific in the implementation of these
>> functions.  i think the way we should be handling these is by doing:
>>  - remove {lock,unlock}_bus functions from spi_master
>>  - move the {lock,unlock}_bus code from spi_bfin5xx.c to spi.c
>>  - drop the SPI_BFIN_LOCK Kconfig
>
> Am I missing something. I cannot find any lock related functions in
> the spi code.  Is this stuff in mainline?

i'm proposing what should be done.  there is no locking logic anywhere atm.
-mike

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