linux-kernel - Re: [PATCH 1/1] mct_u232: added _ioctl, _msr_to_icount and _get

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Date:	Tue, 28 Dec 2010 07:15:35 -0800 (PST)
From:	Tsozik <tsozik@...oo.com>
To:	Greg Kroah-Hartman <gregkh@...e.de>,
	Pete Zaitcev <zaitcev@...hat.com>
Cc:	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	zaitcev@...hat.com
Subject: Re: [PATCH 1/1] mct_u232: added _ioctl, _msr_to_icount and _get_icount functions

Greg,

I'm sorry to bother you again, but I'm wondering if you could comment on Pete's concern below.

Thank you in advance for your expertise on the matter,
 Vadim. 

--- On Tue, 12/28/10, Pete Zaitcev <zaitcev@...hat.com> wrote:

> From: Pete Zaitcev <zaitcev@...hat.com>
> Subject: Re: [PATCH 1/1] mct_u232: added _ioctl, _msr_to_icount and _get_icount functions
> To: "Tsozik" <tsozik@...oo.com>
> Cc: "Greg Kroah-Hartman" <gregkh@...e.de>, linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org, zaitcev@...hat.com
> Date: Tuesday, December 28, 2010, 1:40 AM
> On Mon, 27 Dec 2010 20:04:51 -0800
> (PST)
> Tsozik <tsozik@...oo.com>
> wrote:
> 
> > So I ran geiger counter against /dev/ttyS0 device for
> 20 minutes and
> > acquired 20 measurements. Then I compared last average
> with last 20
> > minute measurement average acquired via mct_u232 on
> the laptop placed
> > nearby. The error was ~4% (rounded up).
> 
> Great, I'm ready to ack.
> 
> There's just one thing that is bugging me... I think it
> would be best
> if Alan Cox or Greg Kroah commented on it. The edgeport
> does the
> following, which we copied:
> 
> 
>         schedule();
>         ........
>         if (cnow.rng ==
> cprev.rng && cnow.dsr == cprev.dsr &&
>            
> cnow.dcd == cprev.dcd && cnow.cts == cprev.cts)
>            
> return -EIO; /* no change => error */
>         if (((arg &
> TIOCM_RNG) && (cnow.rng != cprev.rng)) ||
>             ((arg
> & TIOCM_DSR) && (cnow.dsr != cprev.dsr)) ||
>             ((arg
> & TIOCM_CD)  && (cnow.dcd != cprev.dcd))
> ||
>             ((arg
> & TIOCM_CTS) && (cnow.cts != cprev.cts))) {
>            
> return 0;
>         }
> 
> So, if there was a status report, but no change to bits,
> the ioctl
> TIOCMIWAIT would return with -EIO. In serial_core.c, that
> serves
> conventional non-USB UARTs, nothing like this occurs. I am
> not quite
> sure what the point of doing this -EIO check is.
> 
> Oh and BTW, I'm wondering what is going to happen if the
> device is
> disconnected while an application is blocked waiting for
> the status
> change. The patch is not particularly bad here, it just
> copies
> an existing code from elsewhere.
> 
> -- Pete
> 


      
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