Re: [PATCH tty-next v3 1/6] serial: 8250: Adjust the timeout for FIFO mode

[Maciej W. Rozycki]

On Thu, 31 Oct 2024, John Ogness wrote:

> >> THRE only signals there is a space for one character.
> >
> >  Nope[1]:
> >
> > "In the FIFO mode, THRE is set when the transmit FIFO is empty; it is 
> > cleared when at least one byte is written to the transmit FIFO."
> >
> > It seems common enough a misconception that once I actually had to fix the 
> > bad interpretation of THRE in an unpublished platform driver to get decent 
> > performance out of it at higher rates such as 230400bps, as it only pushed 
> > one byte at a time to the FIFO while it had it all available once THRE has 
> > been set.
> 
> I do not know if this is true for all 8250-variants. If there is some
> variant where it functions as Jiri expected, then it would mean
> significant text loss during longer messages. But that would already be
> a problem in the current mainline driver.

 Or rather in my case it would prevent communication from working at all; 
I actually had to fix the issue for networking over a serial line rather 
than just exchanging text messages, and hence a particular need to make it 
run fast.

 I don't expect any 550 clone to work in a different manner, but I find 
the TI manual particularly unambiguous and well-written, and also old 
enough for the 550 to be the state of the art rather than just legacy.

  Maciej