crossystem: arm: Retry if we fail to read a GPIO

It turns out that if two processes try to read the same GPIO at the
same time that one of them will fail because the GPIO is
"busy".

This is really by design of the kernel's GPIO API. In order to read a
value, each userspace process "requests" control of the GPIO and then
queries it. There doesn't appear to be any way to "wait" for a GPIO
that's been requested by someone else--we just need to wait a bit and
try again later. ...so that's what we'll do.

Without this patch, if you run the following script on a
write-protected sc7180-trogdor class device in two shells at the same
time:

  old_val=""
  while true; do
    val=$(crossystem wpsw_cur)
    if [[ "${val}" != "${old_val}" ]]; then
      echo "$(date): ${old_val} => ${val}"
      old_val="${val}"
    fi
  done

Then you'll see stuff like this:

  GPIO_GET_LINEHANDLE_IOCTL: Device or resource busy
  Tue Oct 18 11:34:01 PDT 2022: 1 => 0
  Tue Oct 18 11:34:01 PDT 2022: 0 => 1
  GPIO_GET_LINEHANDLE_IOCTL: Device or resource busy
  Tue Oct 18 11:34:01 PDT 2022: 1 => 0
  Tue Oct 18 11:34:01 PDT 2022: 0 => 1

The 0 actually comes from the fact that crossystem falls back to
`GetVdatInt(VDAT_INT_HW_WPSW_BOOT)` if it fails to read the GPIO and
that value isn't initted to anything on trogdor (VDAT_INT_HW_WPSW_BOOT
is deprecated and not populated on trogdor).

It is postulated that the above problem is causing some parts of the
system to get confused about the write protect state of devices.

BRANCH=none
BUG=b:249498455
TEST=Run script in CL commit message and see no errors

Change-Id: I307cdb4e290c27694690a19af60f4697ee0233e4
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3963985
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Julius Werner <jwerner@chromium.org>

1 files changed, 22 insertions, 1 deletions

diff --git a/host/arch/arm/lib/crossystem_arch.c b/host/arch/arm/lib/crossystem_arch.c
index 700622f6..79fb152e 100644
--- a/host/arch/arm/lib/crossystem_arch.c
+++ b/host/arch/arm/lib/crossystem_arch.c
@@ -18,6 +18,7 @@
 #include <sys/ioctl.h>
 #include <sys/wait.h>
 #include <dirent.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <netinet/in.h>
@@ -199,13 +200,33 @@ static int gpioline_read_value(int chip_fd, int idx, bool active_low)
 		.lines = 1,
 	};
 	struct gpiohandle_data data;
+	int trynum;
 	int ret;
 
-	ret = ioctl(chip_fd, GPIO_GET_LINEHANDLE_IOCTL, &request);
+	/*
+	 * If two callers try to read the same GPIO at the same time then
+	 * one of the two will get back EBUSY. There's no great way to
+	 * solve this, so we'll just retry a bunch with a small sleep in
+	 * between.
+	 */
+	for (trynum = 0; true; trynum++) {
+		ret = ioctl(chip_fd, GPIO_GET_LINEHANDLE_IOCTL, &request);
+
+		/*
+		 * Not part of the loop condition so usleep doesn't clobber
+		 * errno (implicitly used by perror).
+		 */
+		if (ret >= 0 || errno != EBUSY || trynum >= 50)
+			break;
+
+		usleep(trynum * 1000);
+	}
+
 	if (ret < 0) {
 		perror("GPIO_GET_LINEHANDLE_IOCTL");
 		return -1;
 	}
+
 	if (request.fd < 0) {
 		fprintf(stderr, "bad LINEHANDLE fd %d\n", request.fd);
 		return -1;