From cf32b71e981ca63e8f349d8585ca2a3583b556e0 Mon Sep 17 00:00:00 2001 From: Ernst Schwab Date: Mon, 28 Jun 2010 17:49:29 -0700 Subject: spi/mmc_spi: SPI bus locking API, using mutex SPI bus locking API to allow exclusive access to the SPI bus, especially, but not limited to, for the mmc_spi driver. Coded according to an outline from Grant Likely; here is his specification (accidentally swapped function names corrected): It requires 3 things to be added to struct spi_master. - 1 Mutex - 1 spin lock - 1 flag. The mutex protects spi_sync, and provides sleeping "for free" The spinlock protects the atomic spi_async call. The flag is set when the lock is obtained, and checked while holding the spinlock in spi_async(). If the flag is checked, then spi_async() must fail immediately. The current runtime API looks like this: spi_async(struct spi_device*, struct spi_message*); spi_sync(struct spi_device*, struct spi_message*); The API needs to be extended to this: spi_async(struct spi_device*, struct spi_message*) spi_sync(struct spi_device*, struct spi_message*) spi_bus_lock(struct spi_master*) /* although struct spi_device* might be easier */ spi_bus_unlock(struct spi_master*) spi_async_locked(struct spi_device*, struct spi_message*) spi_sync_locked(struct spi_device*, struct spi_message*) Drivers can only call the last two if they already hold the spi_master_lock(). spi_bus_lock() obtains the mutex, obtains the spin lock, sets the flag, and releases the spin lock before returning. It doesn't even need to sleep while waiting for "in-flight" spi_transactions to complete because its purpose is to guarantee no additional transactions are added. It does not guarantee that the bus is idle. spi_bus_unlock() clears the flag and releases the mutex, which will wake up any waiters. The difference between spi_async() and spi_async_locked() is that the locked version bypasses the check of the lock flag. Both versions need to obtain the spinlock. The difference between spi_sync() and spi_sync_locked() is that spi_sync() must hold the mutex while enqueuing a new transfer. spi_sync_locked() doesn't because the mutex is already held. Note however that spi_sync must *not* continue to hold the mutex while waiting for the transfer to complete, otherwise only one transfer could be queued up at a time! Almost no code needs to be written. The current spi_async() and spi_sync() can probably be renamed to __spi_async() and __spi_sync() so that spi_async(), spi_sync(), spi_async_locked() and spi_sync_locked() can just become wrappers around the common code. spi_sync() is protected by a mutex because it can sleep spi_async() needs to be protected with a flag and a spinlock because it can be called atomically and must not sleep Signed-off-by: Ernst Schwab [grant.likely@secretlab.ca: use spin_lock_irqsave()] Signed-off-by: Grant Likely Tested-by: Matt Fleming Tested-by: Antonio Ospite --- include/linux/spi/spi.h | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'include/linux/spi/spi.h') diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h index af56071..ae0a528 100644 --- a/include/linux/spi/spi.h +++ b/include/linux/spi/spi.h @@ -262,6 +262,13 @@ struct spi_master { #define SPI_MASTER_NO_RX BIT(1) /* can't do buffer read */ #define SPI_MASTER_NO_TX BIT(2) /* can't do buffer write */ + /* lock and mutex for SPI bus locking */ + spinlock_t bus_lock_spinlock; + struct mutex bus_lock_mutex; + + /* flag indicating that the SPI bus is locked for exclusive use */ + bool bus_lock_flag; + /* Setup mode and clock, etc (spi driver may call many times). * * IMPORTANT: this may be called when transfers to another @@ -542,6 +549,8 @@ static inline void spi_message_free(struct spi_message *m) extern int spi_setup(struct spi_device *spi); extern int spi_async(struct spi_device *spi, struct spi_message *message); +extern int spi_async_locked(struct spi_device *spi, + struct spi_message *message); /*---------------------------------------------------------------------------*/ @@ -551,6 +560,9 @@ extern int 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_sync_locked(struct spi_device *spi, struct spi_message *message); +extern int spi_bus_lock(struct spi_master *master); +extern int spi_bus_unlock(struct spi_master *master); /** * spi_write - SPI synchronous write -- cgit v1.1