linux-kernel - Re: [PATCH v2 1/2] serial: 8250_early: do not save and restore IER in write callback

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Date:	Wed, 28 Oct 2015 10:36:37 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:	linux-serial@...r.kernel.org,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Kevin Cernekee <cernekee@...il.com>,
	Jiri Slaby <jslaby@...e.com>, Rob Herring <robh@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2 1/2] serial: 8250_early: do not save and restore IER in
 write callback

On 10/27/2015 11:27 PM, Masahiro Yamada wrote:
> Hi Peter,
> 
> 2015-10-27 22:54 GMT+09:00 Peter Hurley <peter@...leysoftware.com>:
>> Hi Masahiro,
>>
>> On 10/24/2015 12:17 AM, Masahiro Yamada wrote:
>>> The IER has already been masked in early_serial8250_setup(), there is
>>> no reason to save and restore it every time early_serial8250_write()
>>> is called.
>>>
>>> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>>> ---
>>>
>>>  drivers/tty/serial/8250/8250_early.c | 12 ------------
>>>  1 file changed, 12 deletions(-)
>>>
>>> diff --git a/drivers/tty/serial/8250/8250_early.c b/drivers/tty/serial/8250/8250_early.c
>>> index 7aff3d8..559b681 100644
>>> --- a/drivers/tty/serial/8250/8250_early.c
>>> +++ b/drivers/tty/serial/8250/8250_early.c
>>> @@ -99,20 +99,8 @@ static void __init early_serial8250_write(struct console *console,
>>>  {
>>>       struct earlycon_device *device = console->data;
>>>       struct uart_port *port = &device->port;
>>> -     unsigned int ier;
>>> -
>>> -     /* Save the IER and disable interrupts preserving the UUE bit */
>>> -     ier = serial8250_early_in(port, UART_IER);
>>> -     if (ier)
>>> -             serial8250_early_out(port, UART_IER, ier & UART_IER_UUE);
>>>
>>>       uart_console_write(port, s, count, serial_putc);
>>
>>
>>
>>> -
>>> -     /* Wait for transmitter to become empty and restore the IER */
>>> -     wait_for_xmitr(port);
>>
>> This wait_for_xmitr() change needs to be in patch 2/2.
>>
> 
> 
> Why?

Because if someone reports a regression bisected to this commit,
we won't know if the regression is because we're not saving/restoring IER
or because the transmitter is still running.

Regards,
Peter Hurley

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