Re: [PATCH v2 4/7] serial: 8250: Handle implementations not having TEMT interrupt using em485

[Maarten Brock]

On 2020-05-02 15:49, Lukas Wunner wrote:
> On Thu, Mar 26, 2020 at 12:14:19AM +0100, Heiko Stuebner wrote:
> > Some 8250 ports have a TEMT interrupt but it's not a part of the 8250
> > standard, instead only available on some implementations.
> >
> > The current em485 implementation does not work on ports without it.
> > The only chance to make it work is to loop-read on LSR register.
> >
> > So add UART_CAP_TEMT to mark 8250 uarts having this interrupt,
> > update all current em485 users with that capability and make
> > the stop_tx function loop-read on uarts not having it.
>
> Just to get a better understanding:  According to the 
> Dw_apb_uart_db.pdf
> databook I've found, the UART does have a "THR empty" interrupt.  So 
> you
> get an interrupt once the Transmit Holding Register (and by consequence
> the FIFO) has been drained.  Then what do you need a TEMT interrupt 
> for?
> Why is the THR interrupt not sufficient?

When the Transmit Holding Register is empty, the Transmitter can still 
be
transmitting. And the Driver Enable must be held active during 
transmission.
I would even say it needs to held active during transmission of the stop 
bit,
but I don't believe there is any uart that has an interrupt flag for 
that.
And since the default state for RS485 is '1' anyway it's not that bad.

Maarten