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drivers: can: mcan: manually track available TX buffers

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The MCAN driver operates in TX queue mode (TXBC.TFQM = 1). In this mode
TXFQS.TFQPI returns the first available buffer (usually buffer zero).

Hardware is free to re-use a buffer as soon as TX completes, it does not
have to wait for the matching TX event to be processed.

If a TX completes and that TX buffer is re-used before processing the TX
event, two TX events for the same buffer occur. The first event calls the
second events TX callback, and the second event results in a NULL pointer
exception.

In a "normal" configuration, the TX event ISR will always preempt the
queuing of a TX frame to the same TX buffer.
However, this issue could occur if:
 * Sending a message with ISRs temporarily disabled.
 * The ISR is processed on a different core to the TX call.

The fix is to manually track which TX buffers are available, only freeing
a buffer after the TX event has been processed.

The MCAN user manual states that this is allowed:
"The application may use register TXBRP instead of the Put Index and may
place messages to any Tx Buffer without pending transmission request"

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>
This commit is contained in:
Grant Ramsay 2023-08-03 14:08:00 +12:00 • committed by Fabio Baltieri
commit e9bc195bf4
1 changed files with 11 additions and 12 deletions
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23
drivers/can/can_mcan.c
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@ -822,7 +822,7 @@ int can_mcan_send(const struct device *dev, const struct can_frame *frame, k_tim
#endif /* !CONFIG_CAN_FD_MODE */
.efc = 1U,
};
uint32_t put_idx;
uint32_t put_idx = -1;
uint32_t reg;
int err;
@ -888,16 +888,16 @@ int can_mcan_send(const struct device *dev, const struct can_frame *frame, k_tim
return -EAGAIN;
}
err = can_mcan_read_reg(dev, CAN_MCAN_TXFQS, &reg);
if (err != 0) {
return err;
}
__ASSERT_NO_MSG((reg & CAN_MCAN_TXFQS_TFQF) != CAN_MCAN_TXFQS_TFQF);
k_mutex_lock(&data->tx_mtx, K_FOREVER);
put_idx = FIELD_GET(CAN_MCAN_TXFQS_TFQPI, reg);
/* Acquire a free TX buffer */
for (int i = 0; i < cbs->num_tx; i++) {
if (cbs->tx[i].function == NULL) {
put_idx = i;
break;
}
}
tx_hdr.mm = put_idx;
if ((frame->flags & CAN_FRAME_IDE) != 0U) {
@ -930,14 +930,13 @@ int can_mcan_send(const struct device *dev, const struct can_frame *frame, k_tim
err = can_mcan_write_reg(dev, CAN_MCAN_TXBAR, BIT(put_idx));
if (err != 0) {
goto err_free_tx_cb;
cbs->tx[put_idx].function = NULL;
goto err_unlock;
}
k_mutex_unlock(&data->tx_mtx);
return 0;
err_free_tx_cb:
cbs->tx[put_idx].function = NULL;
err_unlock:
k_mutex_unlock(&data->tx_mtx);
k_sem_give(&data->tx_sem);