Re: [PATCH] serial: imx: reduce RX interrupt frequency

[Sergey Organov <sorganov@gmail.com>]

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

> On 05.01.2022 16:00:34, Sergey Organov wrote:
>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>> 
>> > On Tue, Jan 04, 2022 at 11:32:03AM +0100, Tomasz Moń wrote:
>> >> Triggering RX interrupt for every byte defeats the purpose of aging
>> >> timer and leads to interrupt starvation at high baud rates.
>> >> 
>> >> Increase receiver trigger level to 8 to increase the minimum period
>> >> between RX interrupts to 8 characters time. The tradeoff is increased
>> >> latency. In the worst case scenario, where RX data has intercharacter
>> >> delay slightly less than aging timer (8 characters time), it can take
>> >> up to 63 characters time for the interrupt to be raised since the
>> >> reception of the first character.
>> >
>> > Why can't you do this dynamically based on the baud rate so as to always
>> > work properly for all speeds without increased delays for slower ones?
>> 
>> I don't like the idea of dynamic threshold as I don't think increased
>> complexity is worth it.
>> 
>> In fact the threshold works "properly" on any baud rate, as maximum
>> latency is proportional to the current baud rate, and if somebody does
>> care about *absolute* latency, increasing baud rate is the primary
>> solution.
>
> Nope - this only works if you have both sides under control.... Which is
> not the case in our $CUSTROMER's use case.

Yep, if one can't use primary solution, they need to come up with
something else. Where is that historical "low-latency" bit, by the way?

Thanks,
-- Sergey Organov