From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754622Ab3IZWrB (ORCPT ); Thu, 26 Sep 2013 18:47:01 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:35434 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754268Ab3IZWq5 (ORCPT ); Thu, 26 Sep 2013 18:46:57 -0400 Date: Thu, 26 Sep 2013 15:46:55 -0700 From: Greg Kroah-Hartman To: Tim Kryger Cc: Heikki Krogerus , linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, patches@linaro.org Subject: Re: [PATCH] serial: 8250_dw: Improve unwritable LCR workaround Message-ID: <20130926224655.GA15106@kroah.com> References: <1380069549-9176-1-git-send-email-tim.kryger@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1380069549-9176-1-git-send-email-tim.kryger@linaro.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 24, 2013 at 05:39:09PM -0700, Tim Kryger wrote: > The Designware UART has a limitation where it ignores writes into the > LCR if the UART is busy. The current workaround stashes a copy of the > last written LCR and writes it back down to the hardware if it receives > a special busy interrupt which is raised when a write was ignored. > > Unfortunately, interrupts are typically disabled prior to performing a > sequence of register writes that include the LCR so the point at which > the retry occurs is too late. An example is serial8250_do_set_termios() > where an ignored LCR write results in the baud divisor not being set and > instead a garbage character is sent out the transmitter. > > Furthermore, since serial_port_out() offers no way to indicate failure, > a serious effort must be made to ensure that the LCR is actually updated > before returning back to the caller. This is difficult, however, as a > UART that was busy during the first attempt is likely to still be busy > when a subsequent attempt is made unless some extra action is taken. > > This updated workaround takes the extreme action of clearing the TX/RX > FIFOs and reading the receive buffer before writing down the LCR in the > hope that doing so will force the UART into an idle state. While this > may seem unnecessarily aggressive, writes to the LCR are used to change > the baud rate, parity, stop bit, or data length so the data that may be > lost is likely not important. Admittedly, this is far from ideal but it > seems to be the best that can be done given the hardware limitations. > > Lastly, the revised workaround doesn't touch the LCR in the ISR, so it > avoids the possibility of a "serial8250: too much work for irq" lock up. > This problem is rare in real situations but can be reproduced easily by > wiring up two UARTs and running the following commands. > > # stty -F /dev/ttyS1 echo > # stty -F /dev/ttyS2 echo > # cat /dev/ttyS1 & > [1] 375 > # echo asdf > /dev/ttyS1 > asdf > > [ 27.700000] serial8250: too much work for irq96 > [ 27.700000] serial8250: too much work for irq96 > [ 27.710000] serial8250: too much work for irq96 > [ 27.710000] serial8250: too much work for irq96 > [ 27.720000] serial8250: too much work for irq96 > [ 27.720000] serial8250: too much work for irq96 > [ 27.730000] serial8250: too much work for irq96 > [ 27.730000] serial8250: too much work for irq96 > [ 27.740000] serial8250: too much work for irq96 > > Signed-off-by: Tim Kryger > Reviewed-by: Matt Porter > Reviewed-by: Markus Mayer > --- > drivers/tty/serial/8250/8250_dw.c | 57 +++++++++++++++++++++++++++------------ > 1 file changed, 40 insertions(+), 17 deletions(-) This patch no longer applies to my tty-next tree, can you please refresh it and resend so that I can apply it? thanks, greg k-h