Commit f5a9c77d authored by David Brownell's avatar David Brownell Committed by Linus Torvalds

spi doc updates

Update two points in the SPI interface documentation:

- Update description of the "chip stays selected after message ends"
  mode.  In some cases it's required for correctness; it isn't just a
  performance tweak.  (Yes: to use this mode on mult-device busses, another
  programming interface will be needed.  One draft has been circulated
  already.)

- Clarify spi_setup(), highlighting that callers must ensure that no
  requests are queued (can't change configuration except between I/Os), and
  that the device must be deselected when this returns (which is a key part
  of why it's called during device init).
Signed-off-by: default avatarDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ed456662
...@@ -341,9 +341,14 @@ extern struct spi_master *spi_busnum_to_master(u16 busnum); ...@@ -341,9 +341,14 @@ extern struct spi_master *spi_busnum_to_master(u16 busnum);
* chip transactions together. * chip transactions together.
* *
* (ii) When the transfer is the last one in the message, the chip may * (ii) When the transfer is the last one in the message, the chip may
* stay selected until the next transfer. This is purely a performance * stay selected until the next transfer. On multi-device SPI busses
* hint; the controller driver may need to select a different device * with nothing blocking messages going to other devices, this is just
* for the next message. * a performance hint; starting a message to another device deselects
* this one. But in other cases, this can be used to ensure correctness.
* Some devices need protocol transactions to be built from a series of
* spi_message submissions, where the content of one message is determined
* by the results of previous messages and where the whole transaction
* ends when the chipselect goes intactive.
* *
* The code that submits an spi_message (and its spi_transfers) * The code that submits an spi_message (and its spi_transfers)
* to the lower layers is responsible for managing its memory. * to the lower layers is responsible for managing its memory.
...@@ -480,14 +485,15 @@ static inline void spi_message_free(struct spi_message *m) ...@@ -480,14 +485,15 @@ static inline void spi_message_free(struct spi_message *m)
/** /**
* spi_setup - setup SPI mode and clock rate * spi_setup - setup SPI mode and clock rate
* @spi: the device whose settings are being modified * @spi: the device whose settings are being modified
* Context: can sleep * Context: can sleep, and no requests are queued to the device
* *
* SPI protocol drivers may need to update the transfer mode if the * SPI protocol drivers may need to update the transfer mode if the
* device doesn't work with the mode 0 default. They may likewise need * device doesn't work with its default. They may likewise need
* to update clock rates or word sizes from initial values. This function * to update clock rates or word sizes from initial values. This function
* changes those settings, and must be called from a context that can sleep. * changes those settings, and must be called from a context that can sleep.
* The changes take effect the next time the device is selected and data * Except for SPI_CS_HIGH, which takes effect immediately, the changes take
* is transferred to or from it. * effect the next time the device is selected and data is transferred to
* or from it. When this function returns, the spi device is deselected.
* *
* Note that this call will fail if the protocol driver specifies an option * Note that this call will fail if the protocol driver specifies an option
* that the underlying controller or its driver does not support. For * that the underlying controller or its driver does not support. For
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment