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sbus: add time-based hardening (only for IO and NuttX)

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Since SBUS does not have CRC, we can use timing information to improve
parsing reliability and reject unexpected bytes.
This commit is contained in:
Beat Küng
2018-12-14 12:11:16 +01:00
parent 4cd8fe0a30
commit eda45b4df2
1 changed files with 34 additions and 21 deletions
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src/lib/rc/sbus.cpp
+34 -21
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@@ -52,6 +52,8 @@
#include "common_rc.h" #include "common_rc.h"
#include <drivers/drv_hrt.h> #include <drivers/drv_hrt.h>
using namespace time_literals;
#define SBUS_DEBUG_LEVEL 0 /* Set debug output level */ #define SBUS_DEBUG_LEVEL 0 /* Set debug output level */
#if defined(__PX4_POSIX_OCPOC) #if defined(__PX4_POSIX_OCPOC)
@@ -280,8 +282,21 @@ bool
sbus_input(int sbus_fd, uint16_t *values, uint16_t *num_values, bool *sbus_failsafe, bool *sbus_frame_drop, sbus_input(int sbus_fd, uint16_t *values, uint16_t *num_values, bool *sbus_failsafe, bool *sbus_frame_drop,
uint16_t max_channels) uint16_t max_channels)
{ {
int ret = 1; /*
hrt_abstime now; * Fetch bytes, but no more than we would need to complete
* a complete frame.
*/
uint8_t buf[SBUS_FRAME_SIZE * 2];
int ret = read(sbus_fd, &buf[0], SBUS_FRAME_SIZE);
/* if the read failed for any reason, just give up here */
if (ret < 1) {
return false;
}
const hrt_abstime now = hrt_absolute_time();
#ifdef __PX4_NUTTX /* limit time-based hardening to RTOS's where we have reliable timing */
/* /*
* The S.BUS protocol doesn't provide reliable framing, * The S.BUS protocol doesn't provide reliable framing,
@@ -298,32 +313,30 @@ sbus_input(int sbus_fd, uint16_t *values, uint16_t *num_values, bool *sbus_fails
* provides a degree of protection. Of course, it would be better * provides a degree of protection. Of course, it would be better
* if we didn't drop bytes... * if we didn't drop bytes...
*/ */
now = hrt_absolute_time(); if (now - last_rx_time > 3_ms) {
if (partial_frame_count > 0) {
partial_frame_count = 0;
sbus_decode_state = SBUS2_DECODE_STATE_DESYNC;
#if defined(SBUS_DEBUG_LEVEL) && SBUS_DEBUG_LEVEL > 0
printf("SBUS: RESET (TIME LIM)\n");
#endif
}
}
/* if (partial_frame_count == 0 && buf[0] != SBUS_START_SYMBOL) {
* Fetch bytes, but no more than we would need to complete /* don't bother going through the buffer if we don't get the
* a complete frame. * expected start symbol as a first byte */
*/ sbus_decode_state = SBUS2_DECODE_STATE_DESYNC;
uint8_t buf[SBUS_FRAME_SIZE * 2];
bool sbus_decoded = false;
ret = read(sbus_fd, &buf[0], SBUS_FRAME_SIZE);
/* if the read failed for any reason, just give up here */
if (ret < 1) {
return false; return false;
} }
#endif /* __PX4_NUTTX */
/* /*
* Try to decode something with what we got * Try to decode something with what we got
*/ */
if (sbus_parse(now, &buf[0], ret, values, num_values, sbus_failsafe, return sbus_parse(now, &buf[0], ret, values, num_values, sbus_failsafe,
sbus_frame_drop, &sbus_frame_drops, max_channels)) { sbus_frame_drop, &sbus_frame_drops, max_channels);
sbus_decoded = true;
}
return sbus_decoded;
} }
bool bool