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Date:   Sun, 17 May 2020 19:28:09 +0200
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 17/05/2020 17:04, Heiko Stübner ha scritto:
> Hi Giulio,
> 
> Am Donnerstag, 26. März 2020, 03:02:39 CEST schrieb Giulio Benetti:
>> Il 26/03/2020 01:05, Heiko Stübner ha scritto:
>>> 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?
> 
> Sorry that this was sitting around for over a month.

np at all

> I think there are two problems with this:
> 
> (1) this would break backwards compatibility ... right now the whole code
> just assumes that everyone does support the TEMT interrupt, so adding
> a property to keep it working would break old DTs, which is something that
> should not happen ... I guess one option would be to use the inverse
> no-temt-interrupt
> 
> (2) uarts handled by 8250_of are still identified by their compatible
> though and there is no generic 8250-of compatible, so the
> presence / absence of the temt capability should actually just be
> bound to the relevant compatible.
> 
> 
> So my "gut feeling" is to just keep the current way
> (was expecting temt-capability before anyway) until an uart
> variant without temt comes along

I agree with you, what I was proposing is another thing more to do an
can be done when needed.

Best regards
-- 
Giulio Benetti
CTO

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