On 04/06/2016 12:14 PM, Uwe Kleine-König wrote:
> Hello,
>
> On Wed, Apr 06, 2016 at 11:35:23AM -0700, Peter Hurley wrote:
>> On 04/06/2016 10:48 AM, Uwe Kleine-König wrote:
>>> On Wed, Apr 06, 2016 at 08:23:02AM -0700, Peter Hurley wrote:
>>>> On 04/05/2016 11:20 PM, Uwe Kleine-König wrote:
>>>>> On Tue, Apr 05, 2016 at 09:58:09AM -0700, Peter Hurley wrote:
>>>>>> On 04/05/2016 06:25 AM, Yegor Yefremov wrote:
>>>>>>> On Tue, Apr 5, 2016 at 2:21 PM, Uwe Kleine-König
>>>>>>> <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
>>>>>>>> On Tue, Apr 05, 2016 at 12:32:53PM +0200, Yegor Yefremov wrote:
>>>>>>>>> I've got a kernel crash from kernel robot. If we use UART before
>>>>>>>>> general initialization (earlyprintk), then any call to mctrl API would
>>>>>>>>> result in NULL pointer dereference. One solution would be to check, if
>>>>>>>>> gpios IS_ERR_OR_NULL(). See below:
>>>>>>>>>
>>>>>>>>> --- a/drivers/tty/serial/serial_mctrl_gpio.c
>>>>>>>>> +++ b/drivers/tty/serial/serial_mctrl_gpio.c
>>>>>>>>> @@ -54,6 +54,9 @@ void mctrl_gpio_set(struct mctrl_gpios *gpios,
>>>>>>>>> unsigned int mctrl)
>>>>>>>>> int value_array[UART_GPIO_MAX];
>>>>>>>>> unsigned int count = 0;
>>>>>>>>>
>>>>>>>>> + if (IS_ERR_OR_NULL(gpios))
>>>>>>>>> + return;
>>>>>>>>> +
>>>>>>>>> for (i = 0; i < UART_GPIO_MAX; i++)
>>>>>>>>> if (gpios->gpio[i] && mctrl_gpios_desc[i].dir_out) {
>>>>>>>>> desc_array[count] = gpios->gpio[i];
>>>>>>>>
>>>>>>>> IS_ERR_OR_NULL(gpios) should never be true. gpios should be the value
>>>>>>>> that was returned by mctrl_gpio_init, this never returns NULL and if it
>>>>>>>> returns an error you're supposed to not register the port. And for early
>>>>>>>> printk there is AFAIK no mctrl involved.
>>>>>>>
>>>>>>> You're right. it was console_init stuff. It happens before
>>>>>>> serial8250_register_8250_port(). Perhaps I should introduce one more
>>>>>>> gpio_init invocation in univ8250_console_setup().
>>>>>
>>>>> If the port isn't registered yet, nobody should call the port's
>>>>> .set_mctrl. So your plan sounds wrong for this reason, too.
>>>>
>>>> The 8250 driver initializes MCR from mctrl in its set_termios method:
>>>> uart_set_options
>>>> mctrl |= TIOCM_DTR
>>>> ->set_termios => serial8250_set_termios
>>>> serial8250_set_mctrl
>>>
>>> Then maybe the bug is that uart_set_options calls serial8250_set_mctrl
>>> which is supposed to be only called after the device is probed?
>>
>> Nope; DTR should be asserted when the console is initialized.
>
> Oh, that's news to me. I thought console communication is supposed to
> never use handshaking. Who can give an authorative answer here? Greg?
> Russell?
Since v2.6.23:
commit 79492689e40d4f4d3d8a7262781d56fb295b4b86
Author: Yinghai Lu <Yinghai.Lu@xxxxxxx>
Date: Sun Jul 15 23:37:25 2007 -0700
serial: assert DTR for serial console devices
Some RS-232 devices require DTR to be asserted before they can be used. DTR
is normally asserted in uart_startup() when the port is opened. But we don't
actually open serial console ports, so assert DTR when the port is added.
BTW:
earlyprintk and early_uart are hard coded to set DTR/RTS.
rmk says
The only issue I can think of is the possibility for an attached modem to
auto-answer or maybe even auto-dial before the system is ready for it to do
so. Might have an undesirable cost implication for some running with such a
setup.
Apart from that, I can't think of any other side effect of this specific
patch.
Signed-off-by: Yinghai Lu <yinghai.lu@xxxxxxx>
Acked-by: Russell King <rmk@xxxxxxxxxxxxxxxx>
>> I understand that is not possible with the mctrl helpers right now,
>> but that's no reason to break other setups that do the right thing.
>>
>>>>>> Just skip mctrl_gpio_set() and mctrl_gpio_get*() if !up->gpios
>>>>>
>>>>> This would work, but sounds wrong for the above reason, too. I'd like to
>>>>> reserve gpios=NULL for the case where no gpio has to be controlled, so
>>>>> please don't use it as indication if mctrl_gpio_init was called.
>>>>
>>>> I'm confused; what operations will be different if gpios==NULL?
>>>> And wouldn't that argue for checking gpios==NULL in mctrl_gpio_set(),
>>>> performing no action in that case?
>>>
>>> OK, the right thing would happen. Still I'd prefer if a serial driver
>>> did not try to interpret what a certain value means or not. I'd say the
>>> only allowed operations on a gpios value are calling mctrl_gpio
>>> functions and use IS_ERR and PTR_ERR during probe.
>>
>> Ok, so then we're back to checking gpios == NULL in mctrl_gpio_set()
>> instead, right?
>>
>> Because that's "the case where no gpio has to be controlled" because
>> there is no gpio yet.
>
> I don't agree. It's a layer violation if you pass a "self made" value
> (in this case NULL) to an mctrl_gpio function. Currently mctrl_gpio_init
> cannot return NULL, so it's a bug to call mctrl_gpio_set with NULL.
Which is exactly the opposite argument you just waged 2 emails before.
Ok, so if "mctrl_gpio_init() cannot return NULL", and gpios is a
self-made value, then I see no problem simply not calling
mctrl_gpio_set() if up->gpios is NULL.
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