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Fix inconsistency between mbed crc and psoc6 crc implementations. #12324

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Merged
merged 1 commit into from
Jan 30, 2020
Merged

Fix inconsistency between mbed crc and psoc6 crc implementations. #12324

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18 changes: 17 additions & 1 deletion targets/TARGET_Cypress/TARGET_PSOC6/cy_crc_api.c
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,14 @@ static cyhal_crc_t cy_crc;
static crc_algorithm_t cy_crc_cfg;
static bool cy_crc_initialized = false;

// Reverses width least significant bits of 32 bit input. Any bits more
// significant than width are dropped.
static uint32_t reverse(uint8_t width, uint32_t in)
{
MBED_ASSERT(width <= 32);
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Personally, I wouldn't bother with the assert - this is internal, and you've already cleared the HAL_CRC_IS_SUPPORTED check, and there are a squillion uint32_ts around here. 32-bit or less is kind of assumed.

Actually though, having this after HAL_CRC_IS_SUPPORTED means the compiler should hopefully skip putting in the assert anyway. So maybe I shouldn't be worrying about code size.

return __RBIT(in) >> (32 - width);
}

void hal_crc_compute_partial_start(const crc_mbed_config_t *config)
{
if (!cy_crc_initialized) {
Expand All @@ -46,9 +54,17 @@ void hal_crc_compute_partial_start(const crc_mbed_config_t *config)
cy_crc_cfg.polynomial = config->polynomial;
cy_crc_cfg.lfsrInitState = config->initial_xor;
cy_crc_cfg.dataXor = 0;
cy_crc_cfg.remXor = config->final_xor;
cy_crc_cfg.dataReverse = config->reflect_in ? 1 : 0;
cy_crc_cfg.remReverse = config->reflect_out ? 1 : 0;

// There is an incongruity between what MBeds CRC spec expects and what
// PSoC6 hardware actually performs when it comes to the final XOR and
// remainder reversal: MBed expects the final remainder to be reversed then
// XORed with remXor while PSoC6s CRC, however, performs the final XOR with
// remXor then reverses the resulting value. Since Rev(A) XOR B == Rev( A
// XOR Rev(B) ), a simple fix is to reverse remXor if reflect_out is true.
cy_crc_cfg.remXor = config->reflect_out ? reverse(config->width, config->final_xor) : config->final_xor;

if (CY_RSLT_SUCCESS != cyhal_crc_start(&cy_crc, &cy_crc_cfg)) {
MBED_ERROR(MBED_MAKE_ERROR(MBED_MODULE_DRIVER, MBED_ERROR_CODE_FAILED_OPERATION), "cyhal_crc_start");
}
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