jtag/drivers/bcm2835gpio: add peripheral_mem_dev config command

The bcm2835gpio driver preferred /dev/gpiomem for access to
memory mapped GPIO control and used /dev/mem as a fallback
only if it couldn't open /dev/gpiomem.

/dev/mem usually requires elevated rights or specific capabilities
of the opening process, so the fallback failed anyway.

Although /dev/gpiomem is the strongly preferred option with respect
to security, there could be also use cases which require /dev/mem
even if /dev/gpiomem is available (e.g. changing the GPIO pad
settings is necessary or testing/debugging OpenOCD).
It was difficult to handle such cases because they required
to block globally the system device /dev/gpiomem
(remove, rename or chmod).

Drop the fallback feature and select the memory device
by 'bcm2835gpio peripheral_mem_dev' configuration command.
Use /dev/gpiomem as a default.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Change-Id: I60e427bda795d7a13d55d61443590dd31d694832
Reviewed-on: https://review.openocd.org/c/openocd/+/7350
Tested-by: jenkins
Reviewed-by: Jonathan Bell <jonathan@raspberrypi.com>

doc/openocd.texi

@@ -3292,10 +3292,6 @@ configuration on exit.
 GPIO numbers >= 32 can't be used for performance reasons. GPIO configuration is
 handled by the generic command @ref{adapter gpio, @command{adapter gpio}}.
 
-/dev/gpiomem is preferred for GPIO mapping with fallback to /dev/mem.
-If /dev/mem is used GPIO 0-27 pads are set to the limited
-slew rate and drive strength is reduced to 4 mA (2 mA on RPi 4).
-
 See @file{interface/raspberrypi-native.cfg} for a sample config and
 @file{interface/raspberrypi-gpio-connector.cfg} for pinout.
 
@@ -3304,8 +3300,25 @@ Set SPEED_COEFF and SPEED_OFFSET for delay calculations. If unspecified,
 speed_coeff defaults to 113714, and speed_offset defaults to 28.
 @end deffn
 
+@deffn {Config Command} {bcm2835gpio peripheral_mem_dev} @var{device}
+Set the device path for access to the memory mapped GPIO control registers.
+Uses @file{/dev/gpiomem} by default, this is also the preferred option with
+respect to system security.
+If overridden to @file{/dev/mem}:
+@itemize @minus
+@item OpenOCD needs @code{cap_sys_rawio} or run as root to open @file{/dev/mem}.
+Please be aware of security issues imposed by running OpenOCD with
+elevated user rights and by @file{/dev/mem} itself.
+@item correct @command{peripheral_base} must be configured.
+@item GPIO 0-27 pads are set to the limited slew rate
+and drive strength is reduced to 4 mA (2 mA on RPi 4).
+@end itemize
+
+@end deffn
+
 @deffn {Config Command} {bcm2835gpio peripheral_base} @var{base}
-Set the peripheral base register address to access GPIOs. For the RPi1, use
+Set the peripheral base register address to access GPIOs.
+Ignored if @file{/dev/gpiomem} is used. For the RPi1, use
 0x20000000. For RPi2 and RPi3, use 0x3F000000. For RPi4, use 0xFE000000. A full
 list can be found in the
 @uref{https://www.raspberrypi.org/documentation/hardware/raspberrypi/peripheral_addresses.md, official guide}.

src/jtag/drivers/bcm2835gpio.c

@@ -19,6 +19,7 @@
 
 #include <sys/mman.h>
 
+static char *bcm2835_peri_mem_dev;
 static off_t bcm2835_peri_base = 0x20000000;
 #define BCM2835_GPIO_BASE	(bcm2835_peri_base + 0x200000) /* GPIO controller */
 
@@ -57,6 +58,14 @@ static struct initial_gpio_state {
 } initial_gpio_state[ADAPTER_GPIO_IDX_NUM];
 static uint32_t initial_drive_strength_etc;
 
+static inline const char *bcm2835_get_mem_dev(void)
+{
+	if (bcm2835_peri_mem_dev)
+		return bcm2835_peri_mem_dev;
+
+	return "/dev/gpiomem";
+}
+
 static inline void bcm2835_gpio_synchronize(void)
 {
 	/* Ensure that previous writes to GPIO registers are flushed out of
@@ -300,6 +309,18 @@ COMMAND_HANDLER(bcm2835gpio_handle_speed_coeffs)
 	return ERROR_OK;
 }
 
+COMMAND_HANDLER(bcm2835gpio_handle_peripheral_mem_dev)
+{
+	if (CMD_ARGC == 1) {
+		free(bcm2835_peri_mem_dev);
+		bcm2835_peri_mem_dev = strdup(CMD_ARGV[0]);
+	}
+
+	command_print(CMD, "BCM2835 GPIO: peripheral_mem_dev = %s",
+				  bcm2835_get_mem_dev());
+	return ERROR_OK;
+}
+
 COMMAND_HANDLER(bcm2835gpio_handle_peripheral_base)
 {
 	uint64_t tmp_base;
@@ -322,6 +343,13 @@ static const struct command_registration bcm2835gpio_subcommand_handlers[] = {
 		.help = "SPEED_COEFF and SPEED_OFFSET for delay calculations.",
 		.usage = "[SPEED_COEFF SPEED_OFFSET]",
 	},
+	{
+		.name = "peripheral_mem_dev",
+		.handler = &bcm2835gpio_handle_peripheral_mem_dev,
+		.mode = COMMAND_CONFIG,
+		.help = "device to map memory mapped GPIOs from.",
+		.usage = "[device]",
+	},
 	{
 		.name = "peripheral_base",
 		.handler = &bcm2835gpio_handle_peripheral_base,
@@ -413,16 +441,16 @@ static int bcm2835gpio_init(void)
 		return ERROR_JTAG_INIT_FAILED;
 	}
 
-	bool pad_mapping_possible = false;
+	bool is_gpiomem = strcmp(bcm2835_get_mem_dev(), "/dev/gpiomem") == 0;
+	bool pad_mapping_possible = !is_gpiomem;
 
-	dev_mem_fd = open("/dev/gpiomem", O_RDWR | O_SYNC);
+	dev_mem_fd = open(bcm2835_get_mem_dev(), O_RDWR | O_SYNC);
 	if (dev_mem_fd < 0) {
-		LOG_DEBUG("Cannot open /dev/gpiomem, fallback to /dev/mem");
-		dev_mem_fd = open("/dev/mem", O_RDWR | O_SYNC);
-		pad_mapping_possible = true;
-	}
-	if (dev_mem_fd < 0) {
-		LOG_ERROR("open: %s", strerror(errno));
+		LOG_ERROR("open %s: %s", bcm2835_get_mem_dev(), strerror(errno));
+		/* TODO: add /dev/mem specific doc and refer to it
+		 * if (!is_gpiomem && (errno == EACCES || errno == EPERM))
+		 *	LOG_INFO("Consult the user's guide chapter 4.? how to set permissions and capabilities");
+		 */
 		return ERROR_JTAG_INIT_FAILED;
 	}
 
@@ -532,6 +560,7 @@ static int bcm2835gpio_quit(void)
 		pads_base[BCM2835_PADS_GPIO_0_27_OFFSET] = 0x5A000000 | initial_drive_strength_etc;
 	}
 	bcm2835gpio_munmap();
+	free(bcm2835_peri_mem_dev);
 
 	return ERROR_OK;
 }