Add an I/O tracing feature

When debugging drivers it is useful to see what I/O accesses were done
and in what order.

Even if the individual accesses are of little interest it can be useful to
verify that the access pattern is consistent each time an operation is
performed. In this case a checksum can be used to characterise the operation
of a driver. The checksum can be compared across different runs of the
operation to verify that the driver is working properly.

In particular, when performing major refactoring of the driver, where the
access pattern should not change, the checksum provides assurance that the
refactoring work has not broken the driver.

Add an I/O tracing feature and associated commands to provide this facility.
It works by sneaking into the io.h heder for an architecture and redirecting
I/O accesses through its tracing mechanism.

For now no commands are provided to examine the trace buffer. The format is
fairly simple, so 'md' is a reasonable substitute.

Note: The checksum feature is only useful for I/O regions where the contents
do not change outside of software control. Where this is not suitable you can
fall back to manually comparing the addresses. It might be useful to enhance
tracing to only checksum the accesses and not the data read/written.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/README b/README
index 7129df822c..d0723701c8 100644
--- a/README
+++ b/README
@@ -1000,6 +1000,7 @@ The following options need to be configured:
 		CONFIG_CMD_IMLS		  List all images found in NOR flash
 		CONFIG_CMD_IMLS_NAND	* List all images found in NAND flash
 		CONFIG_CMD_IMMAP	* IMMR dump support
+		CONFIG_CMD_IOTRACE	* I/O tracing for debugging
 		CONFIG_CMD_IMPORTENV	* import an environment
 		CONFIG_CMD_INI		* import data from an ini file into the env
 		CONFIG_CMD_IRQ		* irqinfo
@@ -1171,6 +1172,28 @@ The following options need to be configured:
 		Note that if the GPIO device uses I2C, then the I2C interface
 		must also be configured. See I2C Support, below.
 
+- I/O tracing:
+		When CONFIG_IO_TRACE is selected, U-Boot intercepts all I/O
+		accesses and can checksum them or write a list of them out
+		to memory. See the 'iotrace' command for details. This is
+		useful for testing device drivers since it can confirm that
+		the driver behaves the same way before and after a code
+		change. Currently this is supported on sandbox and arm. To
+		add support for your architecture, add '#include <iotrace.h>'
+		to the bottom of arch/<arch>/include/asm/io.h and test.
+
+		Example output from the 'iotrace stats' command is below.
+		Note that if the trace buffer is exhausted, the checksum will
+		still continue to operate.
+
+			iotrace is enabled
+			Start:  10000000	(buffer start address)
+			Size:   00010000	(buffer size)
+			Offset: 00000120	(current buffer offset)
+			Output: 10000120	(start + offset)
+			Count:  00000018	(number of trace records)
+			CRC32:  9526fb66	(CRC32 of all trace records)
+
 - Timestamp Support:
 
 		When CONFIG_TIMESTAMP is selected, the timestamp