Hi,
On Mon, Jun 24, 2024 at 6:31 AM Johan Hovold johan+linaro@kernel.org wrote:
The Qualcomm GENI serial driver does not handle buffer flushing and outputs garbage (or NUL) characters for the remainder of any active TX command after the write buffer has been cleared.
Implement the flush_buffer() callback and use it to cancel any active TX command when the write buffer has been emptied.
I could be reading it wrong, but in the kernel-doc of `struct tty_ldisc_ops` it seems to indicate that flush_buffer() is for the other direction. Specifically, it says:
This function instructs the line discipline to clear its buffers of any input characters it may have queued to be delivered to the user mode process.
It's hard to figure out which direction that matches to, but looking at the descriptions of "read" and "write" makes me believe that it's supposed to flush characters that have been read by the UART, not characters that are being written out to the UART. Maybe I'm misunderstanding or the kernel doc is wrong/incomplete?
I guess the underlying worry I have is that there's no guarantee that the flush function will be called when the kfifo loses bytes. If it ever happens we'll fall back to writing NUL bytes out and that doesn't seem amazing to me. To me it feels like qcom_geni_serial_send_chunk_fifo() should detect this situation and then it should be responsible for canceling, though better (in my mind) is if we never initiate any big transfers if we can get away with that and still be performant.
-Doug