linux-kernel - Re: [PATCH V2] serial/uart: Add tunable RX interrupt trigger I/F of FIFO buffers

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Date:	Tue, 18 Feb 2014 13:11:45 +0900
From:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH V2] serial/uart: Add tunable RX interrupt trigger I/F
 of FIFO buffers

Hi Alan,

Thank you for your reply.

(2014/02/18 0:12), One Thousand Gnomes wrote:
>> According to HW specification of serial UART devices, RX interrupt trigger
>> can be changed, but the trigger is hard-coded. For example, RX interrupt trigger
>> in 16550A can be set to 1, 4, 8, or 14 bytes for HW, but current driver sets
>> the trigger to only 8bytes.
>>
>> This patch makes a 16550A device change RX interrupt trigger from userland.
>
> We probably need to be much smarter about how we set this as lots of non
> 16550A type devices have a tunable trigger.
>
> We tune it dynamically based upon baud rate for many platforms, and we
> turn it on and off according to low latency settings. In other words we
> can't just have an ioctl poking values about in parallel and without any
> locking.

You're right. If a user sets low_latency using ioctl() on an 8250
device, serial8250_clear_fifos() initializing FCR will be executed.
So, my approach which does not get lock is inappropriate.

> For the API IMHO this really belongs in sysfs IMHO. That does mean we need
> to turn the existing sysfs API read/write by adding a few set methods not
> just get ones in drivers/tty/serial/ but it would be a lot cleaner and
> the uart_set_info path does the correct stopping and restarting as well
> as locking for the fifo size change.

As you say, sysfs is more convenient than ioctl for users. Moreover,
using uart_set_info() solves the competent problem. So, I'll
implement this feature in sysfs.

Thank you,
Yoshihiro YUNOMAE

-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@...achi.com


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