linux-kernel - Re: [PATCH] Fix Corruption issue in USB ftdi driver drivers/usb/serial/ftdi

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Date:	Tue, 1 Nov 2011 11:36:07 +0000
From:	Alan Cox <alan@...ux.intel.com>
To:	Andrew Worsley <amworsley@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Uwe Bonnes <bon@...ktron.ikp.physik.tu-darmstadt.de>,
	Johan Hovold <jhovold@...il.com>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix Corruption issue in USB ftdi driver
 drivers/usb/serial/ftdi_sio.c

> I don't know why it repeatedly tries to set this all the time - it
> would appear to be quite a lot of work so perhaps there is something
> else that could be cleaned up. This was the simplest safe change that
> fixed my problem. It appears this code hasn't changed very much since
> the first history information in git that I could see - so perhaps
> nobody else is really noticing this issue for some reason?

It could be a problem specific to some firmware or revision. We've had
a similar quirk with a different USB adapter. The actual calls to keep
changing it are coming from your application however.

>         cflag = termios->c_cflag;
> 
> -       /* FIXME -For this cut I don't care if the line is really
> changing or
> -          not  - so just do the change regardless  - should be able
> to
> -          compare old_termios and tty->termios */
> +       /* compare old_termios and tty->termios */
> +    if (old_termios->c_cflag == termios->c_cflag)
> +        goto no_c_cflag_changes;

You can't do it this way because the speed data is not entirely within
c_cflag. Check c_ispeed and c_ospeed match and for the parity if you want to skip
that check if the parity bits change specifically.

Alan
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