From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3269BC433DF for ; Tue, 11 Aug 2020 14:58:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 04CCA20782 for ; Tue, 11 Aug 2020 14:58:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597157890; bh=rpjJYDNyUBJye/pKJy8+A0p0IjypBh/1eTDNkuQVops=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=udlW5JSH/RYB1Pu/W+UA8DqMrzPwg0kip0Vm8v3c0S+GM8bOoUi9ceqley59Vkzo/ Lv3FogjeTZUvt3AozgfBxe3FMyZObl2SjL2Kpn1e/m6cOWVM37BuYBr3DGhjQHhbTE 8RKGLONDgd3KbFoWkTzqFBThMNIbNzqx83sqC23o= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728801AbgHKO6J (ORCPT ); Tue, 11 Aug 2020 10:58:09 -0400 Received: from mail.kernel.org ([198.145.29.99]:57856 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728788AbgHKO6I (ORCPT ); Tue, 11 Aug 2020 10:58:08 -0400 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E5D872076C; Tue, 11 Aug 2020 14:58:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597157887; bh=rpjJYDNyUBJye/pKJy8+A0p0IjypBh/1eTDNkuQVops=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=HKhalOQMVaEdBkBATPK3DlaVPy3dsufQphPHD8qF1tXlhOeoUlrWjyw9RvLs3Yqx/ R8vgpRuviQTFGJZ8KVdnpRtGlNtOIwsirOSFeDfIFXshjHzDVq1nnHrOnpg5FaCE3C 7jfYOkaLBoTQBGGebtEeUf1liV8Q3b7Ld5r1gPzs= Date: Tue, 11 Aug 2020 16:58:16 +0200 From: Greg Kroah-Hartman To: Sumit Garg Cc: Daniel Thompson , Douglas Anderson , linux-serial@vger.kernel.org, kgdb-bugreport@lists.sourceforge.net, Jiri Slaby , Russell King - ARM Linux admin , Jason Wessel , Linux Kernel Mailing List , linux-arm-kernel Subject: Re: [RFC 0/5] Introduce NMI aware serial drivers Message-ID: <20200811145816.GA424033@kroah.com> References: <1595333413-30052-1-git-send-email-sumit.garg@linaro.org> <20200811135801.GA416071@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-serial-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org On Tue, Aug 11, 2020 at 07:59:24PM +0530, Sumit Garg wrote: > Hi Greg, > > Thanks for your comments. > > On Tue, 11 Aug 2020 at 19:27, Greg Kroah-Hartman > wrote: > > > > On Tue, Aug 11, 2020 at 07:20:26PM +0530, Sumit Garg wrote: > > > On Tue, 21 Jul 2020 at 17:40, Sumit Garg wrote: > > > > > > > > Make it possible for UARTs to trigger magic sysrq from an NMI. With the > > > > advent of pseudo NMIs on arm64 it became quite generic to request serial > > > > device interrupt as an NMI rather than IRQ. And having NMI driven serial > > > > RX will allow us to trigger magic sysrq as an NMI and hence drop into > > > > kernel debugger in NMI context. > > > > > > > > The major use-case is to add NMI debugging capabilities to the kernel > > > > in order to debug scenarios such as: > > > > - Primary CPU is stuck in deadlock with interrupts disabled and hence > > > > doesn't honor serial device interrupt. So having magic sysrq triggered > > > > as an NMI is helpful for debugging. > > > > - Always enabled NMI based magic sysrq irrespective of whether the serial > > > > TTY port is active or not. > > > > > > > > Currently there is an existing kgdb NMI serial driver which provides > > > > partial implementation in upstream to have a separate ttyNMI0 port but > > > > that remained in silos with the serial core/drivers which made it a bit > > > > odd to enable using serial device interrupt and hence remained unused. It > > > > seems to be clearly intended to avoid almost all custom NMI changes to > > > > the UART driver. > > > > > > > > But this patch-set allows the serial core/drivers to be NMI aware which > > > > in turn provides NMI debugging capabilities via magic sysrq and hence > > > > there is no specific reason to keep this special driver. So remove it > > > > instead. > > > > > > > > Approach: > > > > --------- > > > > > > > > The overall idea is to intercept serial RX characters in NMI context, if > > > > those are specific to magic sysrq then allow corresponding handler to run > > > > in NMI context. Otherwise, defer all other RX and TX operations onto IRQ > > > > work queue in order to run those in normal interrupt context. > > > > > > > > This approach is demonstrated using amba-pl011 driver. > > > > > > > > Patch-wise description: > > > > ----------------------- > > > > > > > > Patch #1 prepares magic sysrq handler to be NMI aware. > > > > Patch #2 adds NMI framework to serial core. > > > > Patch #3 and #4 demonstrates NMI aware uart port using amba-pl011 driver. > > > > Patch #5 removes kgdb NMI serial driver. > > > > > > > > Goal of this RFC: > > > > ----------------- > > > > > > > > My main reason for sharing this as an RFC is to help decide whether or > > > > not to continue with this approach. The next step for me would to port > > > > the work to a system with an 8250 UART. > > > > > > > > > > A gentle reminder to seek feedback on this series. > > > > It's the middle of the merge window, and I can't do anything. > > > > Also, I almost never review RFC patches as I have have way too many > > patches that people think are "right" to review first... > > > > Okay, I understand and I can definitely wait for your feedback. My feedback here is this: > > I suggest you work to flesh this out first and submit something that you > > feels works properly. :) > IIUC, in order to make this approach substantial I need to make it > work with 8250 UART (major serial driver), correct? As currently it > works properly for amba-pl011 driver. Yes, try to do that, or better yet, make it work with all serial drivers automatically. thanks, greg k-h