From: Lukas Wunner <lukas@wunner.de>
To: Heiko Stuebner <heiko@sntech.de>
Cc: 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>
Subject: Re: [PATCH v2 4/7] serial: 8250: Handle implementations not having TEMT interrupt using em485
Date: Sat, 2 May 2020 15:49:27 +0200 [thread overview]
Message-ID: <20200502134927.6sb7f3na3ff3rpoa@wunner.de> (raw)
In-Reply-To: <20200325231422.1502366-5-heiko@sntech.de>
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?
> @@ -1529,11 +1535,22 @@ static inline void __stop_tx(struct uart_8250_port *p)
> /*
> * To provide required timeing and allow FIFO transfer,
> * __stop_tx_rs485() must be called only when both FIFO and
> - * shift register are empty. It is for device driver to enable
> - * interrupt on TEMT.
> + * shift register are empty. If 8250 port supports it,
> + * it is for device driver to enable interrupt on TEMT.
> + * Otherwise must loop-read until TEMT and THRE flags are set.
> */
> - if ((lsr & BOTH_EMPTY) != BOTH_EMPTY)
> - return;
> + if (p->capabilities & UART_CAP_TEMT) {
> + if ((lsr & BOTH_EMPTY) != BOTH_EMPTY)
> + return;
> + } else {
> + int lsr;
> +
> + if (readx_poll_timeout(__get_lsr, p, lsr,
> + (lsr & BOTH_EMPTY) == BOTH_EMPTY,
> + 0, 10000) < 0)
> + pr_warn("%s: timeout waiting for fifos to empty\n",
> + p->port.name);
> + }
Do you actually need to check for the timeout? How could this happen?
Only if some other part of the driver would disable the transmitter
I guess, which would be a bug.
Also, note that __stop_tx() may be called from hardirq context via
serial8250_tx_chars(). If the baudrate is low, you may spin for a
fairly long time in IRQ context. E.g. with 9600 8N1, it takes about
1 msec for one char to transmit.
Thanks,
Lukas
next prev parent reply other threads:[~2020-05-02 13:49 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 [this message]
2020-05-05 16:38 ` Maarten Brock
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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