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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 B2E83C433DF for ; Wed, 12 Aug 2020 05:48:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9380820781 for ; Wed, 12 Aug 2020 05:48:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linaro.org header.i=@linaro.org header.b="t3V+ZjoH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726685AbgHLFsj (ORCPT ); Wed, 12 Aug 2020 01:48:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51104 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726567AbgHLFsh (ORCPT ); Wed, 12 Aug 2020 01:48:37 -0400 Received: from mail-lj1-x241.google.com (mail-lj1-x241.google.com [IPv6:2a00:1450:4864:20::241]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 253A6C061787 for ; Tue, 11 Aug 2020 22:48:37 -0700 (PDT) Received: by mail-lj1-x241.google.com with SMTP id h19so844385ljg.13 for ; Tue, 11 Aug 2020 22:48:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=0zEyjemzrW6tuSgaZxYukJPqD4TDeOcf1QxIdZ2ERWE=; b=t3V+ZjoHihxGen/+fgcb+Sl03t+g32bqWe2dXV0gZ0ay5zDsCBaKQXzaAsXiaWlX4M bWnc6EU04acKEt4FPPmZIbGLNgPs5+7PI/M1HLrVIKJCwsyAKf+T8wh9dUqU4Pm5eEev QUX8yIytpIJ9M++9s670b38UU7MBDjMJ4ZRy9uV2/p5zLmLTIsvfYdAMe2lx0r/pv2tr WvI8EyZ0wO9Zz4TYPOT1Ghp3FeHCTh2o4yCQqqN9NuHTgSKAPHEKK4Bi2Dmh9Jk+L/hi mvKVcbex+IDJGph6gi02H2S+N8Qrj6V6jMbx+9fo9F3b6pUnSyZXdf2kOTqQmoqT1mt/ xBKg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=0zEyjemzrW6tuSgaZxYukJPqD4TDeOcf1QxIdZ2ERWE=; b=U1q4myx0lYVEJvG+fKvHJjmfz2UTpdAXXjQaRljiYiWf/dqlrgLRRsKdTLcZIahghN yivF2PWJtzzJRNLpIC8UaFf6UEnT+kFgB2WyiIw5rAAFqpcNYQ9rprATxeaB1Pgf561O VnJaMKiIIzPptYZY8HAZbd7SGe8VY7UMFal6dKVWY97UlL8B6GBHkk5bW5dLoRHWvxQh vkOtmdILyGULEY0rySxE6DazvoI9V4yC9ip1tZiSbQX1iUPw4O5R82DE0BY6Gc+hn9KI IjMmL2WqyOLd4KtdSaEkK8trDkKB8Qaja6nBBLVMvQZ/bmCUAA2ZfUyPRqFMYrOXRfkb mCwQ== X-Gm-Message-State: AOAM533fL9vXnGEdtRgDBsLhx7HVFrtfNImvvLfEj3xyEJ+D5vzZmQWv qb+97HSk4yW4dtyi0aGUarMG81JhoptJ9Z72tGweyg== X-Google-Smtp-Source: ABdhPJwHYv2n245El0sBHY3SXi/OZuUM7Hs+PCZPO5C3+U+GeaumrLCmLEKIDi60P/Ex2ud9bVj2jZmm4c++3mhnB+c= X-Received: by 2002:a05:651c:1293:: with SMTP id 19mr4299748ljc.427.1597211315219; Tue, 11 Aug 2020 22:48:35 -0700 (PDT) MIME-Version: 1.0 References: <1595333413-30052-1-git-send-email-sumit.garg@linaro.org> <20200811135801.GA416071@kroah.com> <20200811145816.GA424033@kroah.com> In-Reply-To: <20200811145816.GA424033@kroah.com> From: Sumit Garg Date: Wed, 12 Aug 2020 11:18:23 +0530 Message-ID: Subject: Re: [RFC 0/5] Introduce NMI aware serial drivers To: Greg Kroah-Hartman 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-serial-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org On Tue, 11 Aug 2020 at 20:28, Greg Kroah-Hartman wrote: > > 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. I would like to make serial drivers work automatically but unfortunately the interrupt request/ handling code is pretty specific to the corresponding serial driver. BTW, I will look for ways how we can make it much easier for serial drivers to adapt. -Sumit > > thanks, > > greg k-h