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

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Date:   Tue, 05 May 2020 18:38:54 +0200
From:   Maarten Brock <m.brock@...mierlo.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Heiko Stuebner <heiko@...ech.de>, gregkh@...uxfoundation.org,
        jslaby@...e.com, andriy.shevchenko@...ux.intel.com,
        matwey.kornilov@...il.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        christoph.muellner@...obroma-systems.com,
        giulio.benetti@...ronovasrl.com,
        Heiko Stuebner <heiko.stuebner@...obroma-systems.com>,
        linux-serial-owner@...r.kernel.org
Subject: Re: [PATCH v2 4/7] serial: 8250: Handle implementations not having
 TEMT interrupt using em485

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