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-rw-r--r--Documentation/spi/spi-summary16
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diff --git a/Documentation/spi/spi-summary b/Documentation/spi/spi-summary
index c6152d1..761debf 100644
--- a/Documentation/spi/spi-summary
+++ b/Documentation/spi/spi-summary
@@ -363,6 +363,22 @@ upper boundaries might include sysfs (especially for sensor readings),
the input layer, ALSA, networking, MTD, the character device framework,
or other Linux subsystems.
+Note that there are two types of memory your driver must manage as part
+of interacting with SPI devices.
+
+ - I/O buffers use the usual Linux rules, and must be DMA-safe.
+ You'd normally allocate them from the heap or free page pool.
+ Don't use the stack, or anything that's declared "static".
+
+ - The spi_message and spi_transfer metadata used to glue those
+ I/O buffers into a group of protocol transactions. These can
+ be allocated anywhere it's convenient, including as part of
+ other allocate-once driver data structures. Zero-init these.
+
+If you like, spi_message_alloc() and spi_message_free() convenience
+routines are available to allocate and zero-initialize an spi_message
+with several transfers.
+
How do I write an "SPI Master Controller Driver"?
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