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 03:02:39 +0100
From:   Giulio Benetti <giulio.benetti@...ronovasrl.com>
To:     Heiko Stübner <heiko@...ech.de>
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 Heiko,

Il 26/03/2020 01:05, Heiko Stübner ha scritto:
> 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.

Probably here it's worth introducing a dt boolean property like
"temt-capability", then you set or not UART_CAP_TEMT according to its 
presence in dts. This way all cases are covered and we can act 
completely through dts files.

What about that?

Best regards
-- 
Giulio Benetti
CTO

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