min on cdc r/w

[tswan-quasi]

Describe the PR
Adds a MIN() check for the bufsize on cdc reads and writes. This is to avoid a loop where the bufsize passed to tud_cdc_write() or tud_cdc_read() is 65536 (or a multiple) causing nothing to be written/read.

Additional context
Casting values to uint16_t greater than UINT16_MAX just truncates the upper bits, which means if bufsize ends up being 65536 (or a multiple) it results in the uint16_t cast giving a value of 0, causing no read/write to occur. This then ultimately returns 0 from the top level function, and if the top level application is relying on the return value of something such as tud_cdc_write(), then the program will get stuck sending nothing.

[hathach]

look good, would you mind changing TU_MIN() to tu_min16() i.e

  uint32_t num_read = tu_fifo_read_n(&p_cdc->rx_ff, buffer, tu_min16(bufsize, UINT16_MAX));

[tswan-quasi]

look good, would you mind changing TU_MIN() to tu_min16() i.e

  uint32_t num_read = tu_fifo_read_n(&p_cdc->rx_ff, buffer, tu_min16(bufsize, UINT16_MAX));

I think that would result in the same issue right? Since arg is type uint16_t would just cast bufsize and do the same thing

[hathach]

I think that would result in the same issue right? Since arg is type uint16_t would just cast bufsize and do the same thing

Ah you are right, my bad :)

[hathach]

thank you, we can use tu_min32, but i guess it is not really matter in this case.