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 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
 - add a new spi_master flag to spi.h like SPI_MASTER_HALF_DUPLEX --
SPI_MASTER_LOCK_BUS
 - have spi_bfin5xx.c/bfin_sport_spi.c add that flag to its master setup
 - have the common spi code key off of that flag to return ENOSYS
 - have the mmc_spi code check that bit in the master before falling
back to its hack
-mike

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