davids5 commented on code in PR #11020:
URL: https://github.com/apache/nuttx/pull/11020#discussion_r1371212483
##########
arch/arm/src/imxrt/imxrt_serial.c:
##########
@@ -1373,6 +1373,7 @@ static inline void imxrt_serialout(struct imxrt_uart_s
*priv,
static int imxrt_dma_nextrx(struct imxrt_uart_s *priv)
{
int dmaresidual = imxrt_dmach_getcount(priv->rxdma);
+ DEBUGASSERT(dmaresidual <= RXDMA_BUFFER_SIZE);
Review Comment:
@danielappiagyei-bc
The rxfifo is a ring buffer. Written to by the DMA and read from by code.
N.B `imxrt_dma_nextrx` is only used on the RX side to check if data has been
added by the DMA.
Based on this setup
```
/* Configure for circular DMA reception into the RX FIFO */
config.saddr = priv->uartbase + IMXRT_LPUART_DATA_OFFSET;
config.daddr = (uint32_t) priv->rxfifo;
config.soff = 0;
config.doff = 1;
config.iter = RXDMA_BUFFER_SIZE;
config.flags = EDMA_CONFIG_LINKTYPE_LINKNONE |
EDMA_CONFIG_LOOPDEST |
EDMA_CONFIG_INTHALF |
EDMA_CONFIG_INTMAJOR;
config.ssize = EDMA_8BIT;
config.dsize = EDMA_8BIT;
config.nbytes = 1;
#ifdef CONFIG_KINETIS_EDMA_ELINK
config.linkch = 0;
#endif
```
The DMA reads a byte from IMXRT_LPUART_DATA_OFFSET and writes a byte it to
priv->rxfifo adding 1 to to the destination address register and decrementing
the count register. (That starts at RXDMA_BUFFER_SIZE. ) EDMA_CONFIG_LOOPDEST
causes the destination address register to be reloaded with priv->rxfifo at
count = 0 and the count is reloaded.
Therefore it can not write outside the buffer.
I could image a case were the count is sampled before the reload but I have
not seen it.
Even if it did, the net effect is `imxrt_dma_nextrx` returns 32. But the
only way the return from `imxrt_dma_nextrx` is used is to compare it with the
`priv->rxdmanext` to determine if data is in the ring.
`priv->rxdmanext` is constrained from 0 - RXDMA_BUFFER_SIZE-1; by
```
priv->rxdmanext++;
if (priv->rxdmanext == RXDMA_BUFFER_SIZE)
{
priv->rxdmanext = 0;
}
```
read access is via
```
c = priv->rxfifo[priv->rxdmanext]
```
Not the DMA' index returned from imxrt_dma_nextrx().
Glitch of 32 would just say there is data, the next read, will see the
truth. So the assert !=0 should stay out.
But the nagging question is how do you see 0..
What are your config settings for DMA and LPUART?
Here is what I have
```
CONFIG_IMXRT_EDMA=y
CONFIG_IMXRT_EDMA_EDBG=y
CONFIG_IMXRT_EDMA_ELINK=y
CONFIG_IMXRT_EDMA_NTCD=64\
# ttyS2
CONFIG_LPUART4_RXDMA=y
CONFIG_LPUART4_TXDMA=y
ONFIG_IMXRT_LPUART4=y
CONFIG_LPUART4_BAUD=57600
CONFIG_LPUART4_IFLOWCONTROL=y
CONFIG_LPUART4_OFLOWCONTROL=y
CONFIG_LPUART4_RXBUFSIZE=600
CONFIG_LPUART4_RXDMA=y
CONFIG_LPUART4_TXBUFSIZE=3000
CONFIG_LPUART4_TXDMA=y
```
I am running on this
https://github.com/PX4/PX4-Autopilot/tree/pr-imxrt1170-imxrt1062 branch with
`make nxp_fmurt1170-v1` using `serial_test -s -c -p /dev/ttyS2 -b 3000000` on
the DUT and `./linux-serial-test -s -p
/dev/serial/by-id/usb-FTDI_TTL-234X-3V3_FT2QRM97-if00-port0 -c -b 3000000` on
my linux box.
imxrt_dma_nextrx returns unsigned int. So the DEBUGASSERT ignores the
dmaresidual > 0 term.
Replacing it with
<img width="932" alt="image"
src="https://github.com/apache/nuttx/assets/1945821/46caece0-97f1-4383-a681-1dffb1cf1d7e";>
Will trap the condition. I have a break on the assert firing. But it has
never. 20 minute in.....
<img width="701" alt="image"
src="https://github.com/apache/nuttx/assets/1945821/732f5497-708e-4c53-b56e-0912e4b098fa";>
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