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From: Maarten Brock <m.brock@vanmierlo.com>
To: Lukas Wunner <lukas@wunner.de>
Cc: Heiko Stuebner <heiko@sntech.de>,
	gregkh@linuxfoundation.org, jslaby@suse.com,
	andriy.shevchenko@linux.intel.com, matwey.kornilov@gmail.com,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	christoph.muellner@theobroma-systems.com,
	giulio.benetti@micronovasrl.com,
	Heiko Stuebner <heiko.stuebner@theobroma-systems.com>,
	linux-serial-owner@vger.kernel.org
Subject: Re: [PATCH v2 4/7] serial: 8250: Handle implementations not having TEMT interrupt using em485
Date: Tue, 05 May 2020 18:38:54 +0200	[thread overview]
Message-ID: <523d8e3fa0215f34532b7990a31b0c17@vanmierlo.com> (raw)
In-Reply-To: <20200502134927.6sb7f3na3ff3rpoa@wunner.de>

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


  reply	other threads:[~2020-05-05 16:39 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-25 23:14 [PATCH RFC v2 0/7] serial: 8250: Add rs485 emulation to 8250_dw Heiko Stuebner
2020-03-25 23:14 ` [PATCH DON'T APPLY v2 1/7] serial: Allow uart_get_rs485_mode() to return errno Heiko Stuebner
2020-03-25 23:14 ` [PATCH DON'T APPLY v2 2/7] dt-bindings: serial: Add binding for rs485 bus termination GPIO Heiko Stuebner
2020-03-25 23:14 ` [PATCH DON'T APPLY v2 3/7] serial: 8250: Support " Heiko Stuebner
2020-03-30 14:51   ` Andy Shevchenko
2020-03-25 23:14 ` [PATCH v2 4/7] serial: 8250: Handle implementations not having TEMT interrupt using em485 Heiko Stuebner
2020-03-25 23:47   ` Giulio Benetti
2020-03-26  0:05     ` Heiko Stübner
2020-03-26  2:02       ` Giulio Benetti
2020-05-17 15:04         ` Heiko Stübner
2020-05-17 17:28           ` Giulio Benetti
2020-03-30 14:52   ` Andy Shevchenko
2020-05-02 13:49   ` Lukas Wunner
2020-05-05 16:38     ` Maarten Brock [this message]
2020-05-17 22:01     ` Heiko Stübner
2020-03-25 23:14 ` [PATCH v2 5/7] dt-bindings: serial: Add binding for rs485 receiver enable GPIO Heiko Stuebner
2020-03-30 14:54   ` Andy Shevchenko
2020-05-02 13:51   ` Lukas Wunner
2020-03-25 23:14 ` [PATCH v2 6/7] serial: 8250: Support separate rs485 rx-enable GPIO Heiko Stuebner
2020-03-30 14:56   ` Andy Shevchenko
2020-05-16 20:14   ` Lukas Wunner
2020-03-25 23:14 ` [PATCH v2 7/7] serial: 8250_dw: add em485 support Heiko Stuebner
2020-05-06 15:25   ` Andy Shevchenko

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