Re: [RFC PATCH 1/2] serial: imx: Avoid busy polling for transmitter to become empty

[Esben Haabendal <esben@geanix.com>]

Marc Kleine-Budde <mkl@pengutronix.de> writes:

> On 03.04.2024 17:22:52, Esben Haabendal wrote:
>> Busy polling with readl() is a rather harsh way to wait for a potentially
>> long time.
>
> This read_poll_timeout_atomic() is compiled to an
> imx_uart_readl()/udelay()/cpu_relax() loop. Does the introduction of
> udelay() bring any advantages?

Good point. Probably not. I can set sleep_us 0 to go back to a tight
loop.

>> While there, introduce a 10 ms timeout on this waiting, similar to what
>> many other serial drivers do.
>
> But you don't handle the return value...

True. But this is similar to all the different wait_for_xmitr()
functions, which does basically the same. They are all void, so the
timeout is handled in same happy-go-lucky style.

I think the best we could do would be to show an error message. But
maybe that is not the most sane thing to do to report a problem with
writing error messages. I don't know, but maybe that is why most the
other serial drivers are handling it like this.

In fsl_lpuart.c and uartlite.c a warning message is printed if/when this
timeout occurs. I am fine with doing that here as well...

On a related note. I am unsure if 10 ms is a good choice for timeout. I
picked it because it seems like a common value used in many/most
drivers. But at least some drivers use something like 1 s, which to me
sounds more sane given that we cannot do any meaningful error handling
on timeout.

/Esben