Add BCM2835 (as found in Raspberry Pi) interface driver

This adds support for JTAG programming by bitbanging GPIOs exposed on
the RaspberryPi's expansion header.

Tested by connecting directly to an STM32VLDiscovery board, without any
additional circuity. I observed maximum about 4MHz on the TCK pin with an
old analogue 'scope and about 100kHz when setting the speed to 100kHz.
Busyloop waiting is needed because even with a single 0ns nanosleep call
(with FIFO priority) it lowers the TCK speed to ~30kHz which is way too low
to be useful.

The speed testing with adapter_khz 2000 gave the following results:
sudo chrt -f 1 nice -n -19 ./src/openocd \
     -f interface/raspberrypi-native.cfg \
     -c "set WORKAREASIZE 0x2000" \
     -f target/stm32f1x.cfg -c "adapter_khz 2000"

wrote 131072 bytes from file random.bin in 3.973677s (32.212 KiB/s)
dumped 131072 bytes in 1.445699s (88.538 KiB/s)

This is 3.7 times faster for writing and 14.3 times faster for reading
compared to the generic sysfsgpio driver; probably the writing speed is
limited by the target itself here and reading speed might be considerably
higher too with appropriate connection and a capable target.

BCM2835 name is choosen over BCM2708 because the published peripherals
datasheet uses the particular model name and not family name.

Change-Id: Ib78168be27f53c2a3c88c3dd8154d1190c318c78
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/758
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

1 files changed, 19 insertions, 0 deletions

diff --git a/doc/openocd.texi b/doc/openocd.texi
index 9ea7edf8..51dab187 100644
--- a/doc/openocd.texi
+++ b/doc/openocd.texi
@@ -574,6 +574,9 @@ produced, PDF schematics are easily found and it is easy to make.
 @item @b{at91rm9200}
 @* Like the EP93xx - but an ATMEL AT91RM9200 based solution using the GPIO pins on the chip.
 
+@item @b{bcm2835gpio}
+@* A BCM2835-based board (e.g. Raspberry Pi) using the GPIO pins of the expansion header.
+
 @end itemize
 
 @node About Jim-Tcl
@@ -3048,6 +3051,22 @@ Turn power switch to target on/off.
 No arguments: print status.
 @end deffn
 
+@deffn {Interface Driver} {bcm2835gpio}
+This SoC is present in Raspberry Pi which is a cheap single-board computer
+exposing some GPIOs on its expansion header.
+
+The driver accesses memory-mapped GPIO peripheral registers directly
+for maximum performance, but the only possible race condition is for
+the pins' modes/muxing (which is highly unlikely), so it should be
+able to coexist nicely with both sysfs bitbanging and various
+peripherals' kernel drivers. The driver restores the previous
+configuration on exit.
+
+See @file{interface/raspberrypi-native.cfg} for a sample config and
+pinout.
+
+@end deffn
+
 @section Transport Configuration
 @cindex Transport
 As noted earlier, depending on the version of OpenOCD you use,