From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753308Ab3JBJrR (ORCPT ); Wed, 2 Oct 2013 05:47:17 -0400 Received: from mga02.intel.com ([134.134.136.20]:21796 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752916Ab3JBJrM (ORCPT ); Wed, 2 Oct 2013 05:47:12 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.90,1018,1371106800"; d="scan'208";a="412872377" Date: Wed, 2 Oct 2013 12:47:07 +0300 From: Heikki Krogerus To: Tim Kryger Cc: Greg Kroah-Hartman , linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, patches@linaro.org Subject: Re: [PATCH v2] serial: 8250_dw: Improve unwritable LCR workaround Message-ID: <20131002094707.GA10383@xps8300> References: <1380647888-32473-1-git-send-email-tim.kryger@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1380647888-32473-1-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 Hi, On Tue, Oct 01, 2013 at 10:18:08AM -0700, Tim Kryger wrote: > When configured with UART_16550_COMPATIBLE=NO or in versions prior to > the introduction of this option, the Designware UART will ignore writes > to the LCR if the UART is busy. The current workaround saves a copy of > the last written LCR and re-writes it in the ISR for a special interrupt > that 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 reads back the LCR after each write to confirm > that the new value was accepted by the hardware. Should the hardware > ignore a write, the TX/RX FIFOs are cleared and the receive buffer read > before attempting to rewrite the LCR out of 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 > --- > > Changes in v2: > - Rebased on tty-next > - Updated commit messsage to mention UART_16550_COMPATIBLE > - Removed potentially unnecessary read of LSR and MSR > - Only attempt workaround when LCR write is ignored I'm OK with this. Reviewed-by: Heikki Krogerus Br, -- heikki