On Tue, Feb 16, 2010 at 12:57 PM, Ernst Schwab <[email protected]> wrote:
> From: Yi Li <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Bryan Wu <[email protected]>
> Signed-off-by: Mike Frysinger <[email protected]>
> ---
> 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.
Have I got this wrong?
g.
> +
> +/**
> + * spi_unlock_bus - unlock SPI bus
> + * @spi: device which want to unlock the bus
> + * Context: any
> + *
> + * The caller has called spi_lock_bus() to lock the bus. It calls
> + * spi_unlock_bus() to release the bus so messages for other devices
> + * can be transferred.
> + *
> + * If the caller did not call spi_lock_bus() before, spi_unlock_bus()
> + * should have no effect.
> + *
> + * It returns zero on success, else a negative error code.
> + */
> +int spi_unlock_bus(struct spi_device *spi)
> +{
> + if (spi->master->unlock_bus)
> + return spi->master->unlock_bus(spi);
> + else
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(spi_unlock_bus);
> +
> +/**
> * spi_sync - blocking/synchronous SPI data transfers
> * @spi: device with which data will be exchanged
> * @message: describes the data transfers
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index c47c4b4..c53292c 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -214,6 +214,8 @@ static inline void spi_unregister_driver(struct
> spi_driver *sdrv)
> * the device whose settings are being modified.
> * @transfer: adds a message to the controller's transfer queue.
> * @cleanup: frees controller-specific state
> + * @lock_bus: lock SPI bus for exclusive access
> + * @unlock_bus: unlock SPI bus so other devices can access
> *
> * Each SPI master controller can communicate with one or more @spi_device
> * children. These make a small bus, sharing MOSI, MISO and SCK signals
> @@ -286,6 +288,9 @@ struct spi_master {
>
> /* called on release() to free memory provided by spi_master */
> void (*cleanup)(struct spi_device *spi);
> +
> + int (*lock_bus)(struct spi_device *spi);
> + int (*unlock_bus)(struct spi_device *spi);
> };
>
> static inline void *spi_master_get_devdata(struct spi_master *master)
> @@ -578,6 +583,8 @@ spi_async(struct spi_device *spi, struct spi_message
> *message)
> */
>
> extern int spi_sync(struct spi_device *spi, struct spi_message *message);
> +extern int spi_lock_bus(struct spi_device *spi);
> +extern int spi_unlock_bus(struct spi_device *spi);
>
> /**
> * spi_write - SPI synchronous write
>
>
>
>
>
--
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.
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