i2s_write write data in the buffer

[wouter-van-bastelaere]

stack:

Zephyr on stm32n6 (nucleo_n657x0_q)

When sending a buffer over sai via i2s_write data is written into the buffer.

I expected the TX buffer to be treated as read-only and remain unchanged by the driver.

It's only ever the first 2 numbers.

Why is that?

Example code:

int16_t tx_block_test[96] = {0};

In attachment a minimal version of a project that displays this issue.

[Saket_Om]

Hello @wouter-van-bastelaere 

Try using a buffer declared as const with i2s_write to check if the compiler issues any warnings or errors.

[wouter-van-bastelaere · Accepted solution]

(declaring the buffer as const did raise warnings.)

I found what the issue is.

I2s_write expect's it's buffer to be from the memory slab that was assigned in I2s's config.

And at the end of i2s_write the memory block is freed to the memory slab.

Memory slabs store data about the linked list of all their buffers in the first 4 bytes of empty buffers.

Meaning that when i2s_write finishes the memory slab mistakenly attempts to put it back in it's linked list of buffers. Which writes data into the first 4 bytes.