From patchwork Mon Jun 22 14:05:16 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Pekka Paalanen X-Patchwork-Id: 11617887 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 01516161F for ; Mon, 22 Jun 2020 14:06:12 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D2F47206D7 for ; Mon, 22 Jun 2020 14:06:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="bMCdc/3Y" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D2F47206D7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 8EA3C6E7D9; Mon, 22 Jun 2020 14:06:08 +0000 (UTC) X-Original-To: dri-devel@lists.freedesktop.org Delivered-To: dri-devel@lists.freedesktop.org Received: from mail-lj1-x243.google.com (mail-lj1-x243.google.com [IPv6:2a00:1450:4864:20::243]) by gabe.freedesktop.org (Postfix) with ESMTPS id BB8CD6E7D9 for ; Mon, 22 Jun 2020 14:06:06 +0000 (UTC) Received: by mail-lj1-x243.google.com with SMTP id n24so19404152lji.10 for ; Mon, 22 Jun 2020 07:06:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=SQdJ6aQpwzzyyhmHG8e9bKzHRo0dEtAAGR3HrcwfcHU=; b=bMCdc/3YVP+iAA4NlrMz94Atw/ExNoh7EXHFEbK33kc/yQ6lguc8TkX/ENRrHAOxzA AJ1ih0DKz/xiAbUvmcTgIAX88yw5vw0XlJBxaWYZpKZOhiDM9jNIti9Ci/S5o5/uoOK9 yRbXMImUzwN3O6Tw+lABYwTT9+dX9LkY/DMcnYawAKBNBr6xktYdeoSFODK+SAY/4UhX MEriajliNmWnMxctQJBAbqDSyyrNuPiPE2F2xL1UEsUctR/rDBcGnCsDFW1oJteMabDF 9BCvEEXnQ8bXTHWOs5xsJKAwxJWz9dzrAf6J8HWSHAUvHqLiaDi96jC1J75Su5unPnZ+ LiFw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=SQdJ6aQpwzzyyhmHG8e9bKzHRo0dEtAAGR3HrcwfcHU=; b=St3pctDClD6fRHFX2F8RDZGdse3rwK09t1PeS8HiOChiayqtaGGeYcSVxw1dQQYDcU lTpFGgak3KG4WflRfOgZAj3dma9ZLgxLJ2vCGGj152fkUQm26nIu55Glx/7IaTXOEwGo IIElwBXeVIlP1ilY5r9WTBud+xUqq/E/7U763PrdXA0JBj+gUuulOt1rtI5F3nTgSuA5 6owgdwydLiseH4pFM7vJtfjtJOMK9w93qibRDIimjMiElcSW+SaGsFxkwEvaOyPAHAK/ xhtCqfX8TgKZcpRr7OJBNXGF7tD/9GyaHGS/2F2upWulnm8NZQ15GUA4HSsNTf8NvJvj pQzw== X-Gm-Message-State: AOAM531y+GnECFtEp5MlCW3KmlqVF0n7efbAEMTZUOZUD1pe3Zy7na7p FIOPiqWrqpst7vDBkakTIrt1a6wE X-Google-Smtp-Source: ABdhPJyVoQj6G8ZDsc57+MR05fdc7/5e1sjaY51fb0KWK5APHfvY0k7sNQHOMojotN1yfY37eqbJQQ== X-Received: by 2002:a05:651c:93:: with SMTP id 19mr8994060ljq.245.1592834764679; Mon, 22 Jun 2020 07:06:04 -0700 (PDT) Received: from localhost.localdomain ([194.136.85.206]) by smtp.gmail.com with ESMTPSA id v20sm2885966lfe.46.2020.06.22.07.06.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Jun 2020 07:06:03 -0700 (PDT) From: Pekka Paalanen To: dri-devel@lists.freedesktop.org Subject: [PATCH v4] drm/doc: device hot-unplug for userspace Date: Mon, 22 Jun 2020 17:05:16 +0300 Message-Id: <20200622140516.10830-1-ppaalanen@gmail.com> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200519100649.12053-1-ppaalanen@gmail.com> References: <20200519100649.12053-1-ppaalanen@gmail.com> MIME-Version: 1.0 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Pekka Paalanen , Karol Herbst , Daniel Vetter , Harry Wentland , Dave Airlie , Ben Skeggs , Sean Paul , =?utf-8?q?Chri?= =?utf-8?q?stian_K=C3=B6nig?= Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" From: Pekka Paalanen Set up the expectations on how hot-unplugging a DRM device should look like to userspace. Written by Daniel Vetter's request and largely based on his comments in IRC and from https://lists.freedesktop.org/archives/dri-devel/2020-May/265484.html . A related Wayland protocol change proposal is at https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/35 Signed-off-by: Pekka Paalanen Reviewed-by: Daniel Vetter Cc: Andrey Grodzovsky Cc: Dave Airlie Cc: Sean Paul Cc: Simon Ser Cc: Noralf Trønnes Cc: Ben Skeggs Cc: Christian König Cc: Harry Wentland Cc: Karol Herbst Reviewed-by: Alex Deucher Acked-by: Noralf Trønnes --- Harry and Christian, could one of you ack this on behalf of AMD drivers? Ben or Karol, could you ack on behalf of Nouveau? Noralf, would this work for the tiny drivers etc.? This is only about laying out plans for the future, not about what drivers do today. We'd just like to be sure the goals are reasonable and everyone is aware of the idea. Thanks, pq v4: - two typo fixes (Daniel) v3: - update ENODEV doc (Daniel) - clarify existing vs. new mmaps (Andrey) - split into KMS and render/cross sections (Andrey, Daniel) - open() returns ENXIO (open(2) man page) - ioctls may return ENODEV (Andrey, Daniel) - new wayland-protocols MR v2: - mmap reads/writes undefined (Daniel) - make render ioctl behaviour driver-specific (Daniel) - restructure the mmap paragraphs (Daniel) - chardev minor notes (Simon) - open behaviour (Daniel) - DRM leasing behaviour (Daniel) - added links Disclaimer: I am a userspace developer writing for other userspace developers. I took some liberties in defining what should happen without knowing what is actually possible or what existing drivers already implement. --- Documentation/gpu/drm-uapi.rst | 114 ++++++++++++++++++++++++++++++++- 1 file changed, 113 insertions(+), 1 deletion(-) diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst index 56fec6ed1ad8..b2585ea6a83e 100644 --- a/Documentation/gpu/drm-uapi.rst +++ b/Documentation/gpu/drm-uapi.rst @@ -1,3 +1,5 @@ +.. Copyright 2020 DisplayLink (UK) Ltd. + =================== Userland interfaces =================== @@ -162,6 +164,116 @@ other hand, a driver requires shared state between clients which is visible to user-space and accessible beyond open-file boundaries, they cannot support render nodes. +Device Hot-Unplug +================= + +.. note:: + The following is the plan. Implementation is not there yet + (2020 May). + +Graphics devices (display and/or render) may be connected via USB (e.g. +display adapters or docking stations) or Thunderbolt (e.g. eGPU). An end +user is able to hot-unplug this kind of devices while they are being +used, and expects that the very least the machine does not crash. Any +damage from hot-unplugging a DRM device needs to be limited as much as +possible and userspace must be given the chance to handle it if it wants +to. Ideally, unplugging a DRM device still lets a desktop continue to +run, but that is going to need explicit support throughout the whole +graphics stack: from kernel and userspace drivers, through display +servers, via window system protocols, and in applications and libraries. + +Other scenarios that should lead to the same are: unrecoverable GPU +crash, PCI device disappearing off the bus, or forced unbind of a driver +from the physical device. + +In other words, from userspace perspective everything needs to keep on +working more or less, until userspace stops using the disappeared DRM +device and closes it completely. Userspace will learn of the device +disappearance from the device removed uevent, ioctls returning ENODEV +(or driver-specific ioctls returning driver-specific things), or open() +returning ENXIO. + +Only after userspace has closed all relevant DRM device and dmabuf file +descriptors and removed all mmaps, the DRM driver can tear down its +instance for the device that no longer exists. If the same physical +device somehow comes back in the mean time, it shall be a new DRM +device. + +Similar to PIDs, chardev minor numbers are not recycled immediately. A +new DRM device always picks the next free minor number compared to the +previous one allocated, and wraps around when minor numbers are +exhausted. + +The goal raises at least the following requirements for the kernel and +drivers. + +Requirements for KMS UAPI +------------------------- + +- KMS connectors must change their status to disconnected. + +- Legacy modesets and pageflips, and atomic commits, both real and + TEST_ONLY, and any other ioctls either fail with ENODEV or fake + success. + +- Pending non-blocking KMS operations deliver the DRM events userspace + is expecting. This applies also to ioctls that faked success. + +- open() on a device node whose underlying device has disappeared will + fail with ENXIO. + +- Attempting to create a DRM lease on a disappeared DRM device will + fail with ENODEV. Existing DRM leases remain and work as listed + above. + +Requirements for Render and Cross-Device UAPI +--------------------------------------------- + +- All GPU jobs that can no longer run must have their fences + force-signalled to avoid inflicting hangs on userspace. + The associated error code is ENODEV. + +- Some userspace APIs already define what should happen when the device + disappears (OpenGL, GL ES: `GL_KHR_robustness`_; `Vulkan`_: + VK_ERROR_DEVICE_LOST; etc.). DRM drivers are free to implement this + behaviour the way they see best, e.g. returning failures in + driver-specific ioctls and handling those in userspace drivers, or + rely on uevents, and so on. + +- dmabuf which point to memory that has disappeared will either fail to + import with ENODEV or continue to be successfully imported if it would + have succeeded before the disappearance. See also about memory maps + below for already imported dmabufs. + +- Attempting to import a dmabuf to a disappeared device will either fail + with ENODEV or succeed if it would have succeeded without the + disappearance. + +- open() on a device node whose underlying device has disappeared will + fail with ENXIO. + +.. _GL_KHR_robustness: https://www.khronos.org/registry/OpenGL/extensions/KHR/KHR_robustness.txt +.. _Vulkan: https://www.khronos.org/vulkan/ + +Requirements for Memory Maps +---------------------------- + +Memory maps have further requirements that apply to both existing maps +and maps created after the device has disappeared. If the underlying +memory disappeared, the map is created or modified such that reads and +writes will still complete successfully but the result is undefined. +This applies to both userspace mmap()'d memory and memory pointed to by +dmabuf which might be mapped to other devices (cross-device dmabuf +imports). + +Raising SIGBUS is not an option, because userspace cannot realistically +handle it. Signal handlers are global, which makes them extremely +difficult to use correctly from libraries like those that Mesa produces. +Signal handlers are not composable, you can't have different handlers +for GPU1 and GPU2 from different vendors, and a third handler for +mmapped regular files. Threads cause additional pain with signal +handling as well. + .. _drm_driver_ioctl: IOCTL Support on Device Nodes @@ -199,7 +311,7 @@ EPERM/EACCES: difference between EACCES and EPERM. ENODEV: - The device is not (yet) present or fully initialized. + The device is not anymore present or is not yet fully initialized. EOPNOTSUPP: Feature (like PRIME, modesetting, GEM) is not supported by the driver.