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

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Date:   Thu, 26 Mar 2020 01:05:17 +0100
From:   Heiko Stübner <heiko@...ech.de>
To:     Giulio Benetti <giulio.benetti@...ronovasrl.com>
Cc:     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,
        lukas@...ner.de, christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH v2 4/7] serial: 8250: Handle implementations not having TEMT interrupt using em485

Hi Giulio,

Am Donnerstag, 26. März 2020, 00:47:38 CET schrieb Giulio Benetti:
> very cleaner way to handle TEMT as a capability!
> And I've found one thing...
> 
> Il 26/03/2020 00:14, Heiko Stuebner ha scritto:
> > From: Giulio Benetti <giulio.benetti@...ronovasrl.com>
> > 
> > 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.
> > 
> > Signed-off-by: Giulio Benetti <giulio.benetti@...ronovasrl.com>
> > [moved to use added UART_CAP_TEMT, use readx_poll_timeout]
> > Signed-off-by: Heiko Stuebner <heiko.stuebner@...obroma-systems.com>
> > ---
> >   drivers/tty/serial/8250/8250.h            |  1 +
> >   drivers/tty/serial/8250/8250_bcm2835aux.c |  2 +-
> >   drivers/tty/serial/8250/8250_of.c         |  2 ++
> >   drivers/tty/serial/8250/8250_omap.c       |  2 +-
> >   drivers/tty/serial/8250/8250_port.c       | 25 +++++++++++++++++++----
> >   5 files changed, 26 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> > index 52bb21205bb6..770eb00db497 100644
> > --- a/drivers/tty/serial/8250/8250.h
> > +++ b/drivers/tty/serial/8250/8250.h
> > @@ -82,6 +82,7 @@ struct serial8250_config {
> >   #define UART_CAP_MINI	(1 << 17)	/* Mini UART on BCM283X family lacks:
> >   					 * STOP PARITY EPAR SPAR WLEN5 WLEN6
> >   					 */
> > +#define UART_CAP_TEMT	(1 << 18)	/* UART has TEMT interrupt */
> >   
> >   #define UART_BUG_QUOT	(1 << 0)	/* UART has buggy quot LSB */
> >   #define UART_BUG_TXEN	(1 << 1)	/* UART has buggy TX IIR status */
> > diff --git a/drivers/tty/serial/8250/8250_bcm2835aux.c b/drivers/tty/serial/8250/8250_bcm2835aux.c
> > index 12d03e678295..3881242424ca 100644
> > --- a/drivers/tty/serial/8250/8250_bcm2835aux.c
> > +++ b/drivers/tty/serial/8250/8250_bcm2835aux.c
> > @@ -91,7 +91,7 @@ static int bcm2835aux_serial_probe(struct platform_device *pdev)
> >   		return -ENOMEM;
> >   
> >   	/* initialize data */
> > -	up.capabilities = UART_CAP_FIFO | UART_CAP_MINI;
> > +	up.capabilities = UART_CAP_FIFO | UART_CAP_MINI | UART_CAP_TEMT;
> >   	up.port.dev = &pdev->dev;
> >   	up.port.regshift = 2;
> >   	up.port.type = PORT_16550;
> > diff --git a/drivers/tty/serial/8250/8250_of.c b/drivers/tty/serial/8250/8250_of.c
> > index 65e9045dafe6..841f6fcb2878 100644
> > --- a/drivers/tty/serial/8250/8250_of.c
> > +++ b/drivers/tty/serial/8250/8250_of.c
> > @@ -225,6 +225,8 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
> >   			&port8250.overrun_backoff_time_ms) != 0)
> >   		port8250.overrun_backoff_time_ms = 0;
> >   
> > +	port8250.capabilities |= UART_CAP_TEMT;
> > +
> 
> Shouldn't this be NOT UART_CAP_TEMT set by default? On all other
> vendor specific files you enable it, I think here you shouldn't enable
> it too by default. Right?

8250_of does use the em485 emulation - see of_platform_serial_setup()
So I did go by the lazy assumption that any 8250 driver using rs485
before my series always used the interrupt driver code path, so
implicitly required to have the TEMT interrupt.

Of course, you're right that with the 8250_of maybe not all variants
actually do have this interrupt, so falling back to the polling here might
be safer.


Heiko