[{"id":1537,"web_url":"https://patchwork.libcamera.org/comment/1537/","msgid":"","date":"2019-04-30T07:55:07","subject":"Re: [libcamera-devel] [PATCH 08/11] libcamera: media_device: Add\n\tfunctions to lock device for other processes","submitter":{"id":4,"url":"https://patchwork.libcamera.org/api/people/4/","name":"Kieran Bingham","email":"kieran.bingham@ideasonboard.com"},"content":"Hi Niklas,\n\nOn 29/04/2019 20:17, Niklas Söderlund wrote:\n> Add lock() and unlock() which backed by lockf() allows an instance of\n\n\"which are backed by lockf() and allows an\"\n\n> libcamera to claim exclusive access to a media device. These functions\n> is the base to allow multiple users of libcamera to coexist in the\n\n\"are the base of allowing multiple users...\"\n\n> system without stepping on each others toes.\n> \n\nNice - lockf makes this look pretty easy!\n\n> Signed-off-by: Niklas Söderlund \n\n\n\n\n> ---\n> src/libcamera/include/media_device.h | 4 +++\n> src/libcamera/media_device.cpp | 54 +++++++++++++++++++++++++++-\n> 2 files changed, 57 insertions(+), 1 deletion(-)\n> \n> diff --git a/src/libcamera/include/media_device.h b/src/libcamera/include/media_device.h\n> index e513a2fee1b91a1b..7b88e2875d59dfc3 100644\n> --- a/src/libcamera/include/media_device.h\n> +++ b/src/libcamera/include/media_device.h\n> @@ -30,6 +30,9 @@ public:\n> \tvoid release();\n> \tbool busy() const { return acquired_; }\n> \n> +\tbool lock();\n> +\tvoid unlock();\n> +\n> \tint populate();\n> \tbool valid() const { return valid_; }\n> \n> @@ -58,6 +61,7 @@ private:\n> \tint fd_;\n> \tbool valid_;\n> \tbool acquired_;\n> +\tbool lockOwner_;\n> \n> \tint open();\n> \tvoid close();\n> diff --git a/src/libcamera/media_device.cpp b/src/libcamera/media_device.cpp\n> index cd6a01ed452073e2..9116330570d7b8c2 100644\n> --- a/src/libcamera/media_device.cpp\n> +++ b/src/libcamera/media_device.cpp\n> @@ -63,7 +63,8 @@ LOG_DEFINE_CATEGORY(MediaDevice)\n> * populate() before the media graph can be queried.\n> */\n> MediaDevice::MediaDevice(const std::string &deviceNode)\n> -\t: deviceNode_(deviceNode), fd_(-1), valid_(false), acquired_(false)\n> +\t: deviceNode_(deviceNode), fd_(-1), valid_(false), acquired_(false),\n> +\t lockOwner_(false)\n> {\n> }\n> \n> @@ -118,6 +119,57 @@ void MediaDevice::release()\n> \tacquired_ = false;\n> }\n> \n> +/**\n> + * \\brief Lock the device for use by other instances of libcamera\n> + *\n> + * Multiple instances of libcamera might be running on the same system, at the\n> + * same time. To allow the different instances to coexists system resources in\n\ns/coexists/coexist,/\n\n\n> + * the form of media devices must be accessible for enumerating the cameras\n> + * they provide at all times, while still allowing an instance to lock a\n> + * resource while it prepares to actively use a camera from the resource.\n> + *\n> + * This method shall not be called from a pipeline handler implementation\n> + * directly, as the base PipelineHandler implementation handles this on the\n> + * behalf of the specified implementation.\n> + *\n> + * \\return True if the device could be locked, false otherwise\n> + * \\sa unlock()\n> + */\n> +bool MediaDevice::lock()\n> +{\n> +\tif (fd_ == -1)\n> +\t\treturn false;\n> +\n> +\tif (lockOwner_)\n> +\t\treturn true;\n\nDoes this encourage nested locking?\n\n(I don't yet know if nested locking is a good or bad thing for us)\n\nIf it's something we want to discourage, could we print a debug message\nor warning stating that the file is locked twice perhaps?\n\n\nIf it's something we desire - then I think the nested locks would\nrequire some reference counting and only release on the last unlock call.\n\n\n> +\n> +\tif (lockf(fd_, F_TLOCK, 0))\n> +\t\treturn false;\n> +\n> +\tlockOwner_ = true;\n> +\n> +\treturn true;\n> +}\n> +\n> +/**\n> + * \\brief Unlock the device and free it for use for libcamera instances\n> + *\n> + * This method shall not be called from a pipeline handler implementation\n> + * directly, as the base PipelineHandler implementation handles this on the\n> + * behalf of the specified implementation.\n> + *\n> + * \\sa lock()\n> + */\n> +void MediaDevice::unlock()\n> +{\n> +\tif (fd_ == -1)\n> +\t\treturn;\n> +\n> +\tlockOwner_ = false;\n> +\n\nMediaDevice::lock() \"supports\" being called on a locked file;\n\nWhat happens if you try to unlock an unlocked file? (man lockf does not\nreally detail what happens there). Presumably it will just return an\nerror and take no action?\n\n\n> +\tlockf(fd_, F_ULOCK, 0);\n> +}\n> +\n> /**\n> * \\fn MediaDevice::busy()\n> * \\brief Check if a device is in use\n>","headers":{"Return-Path":"","Received":["from perceval.ideasonboard.com (perceval.ideasonboard.com\n\t[IPv6:2001:4b98:dc2:55:216:3eff:fef7:d647])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTPS id D6DB060E57\n\tfor ;\n\tTue, 30 Apr 2019 09:55:11 +0200 (CEST)","from [192.168.0.20]\n\t(cpc89242-aztw30-2-0-cust488.18-1.cable.virginm.net [86.31.129.233])\n\tby perceval.ideasonboard.com (Postfix) with ESMTPSA id 3048531D;\n\tTue, 30 Apr 2019 09:55:11 +0200 (CEST)"],"DKIM-Signature":"v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com;\n\ts=mail; t=1556610911;\n\tbh=CHiUh9Z5Verib/I7s9Z+HDsX3Xp5vzo18An85UeGZho=;\n\th=Reply-To:Subject:To:References:From:Date:In-Reply-To:From;\n\tb=gEtJlXCt4knCAJQ21b9ugG7hQNkzYCqDbUXtuJmJwF4GkG5ZuNnvqATG4az9G+Ddq\n\tVs/TJh8bQQuVcjWpWc9+OG0XYAqSRugiXKxLrxApiBHMtmrGM72JXhEWXFjIYhh5dM\n\tMRA6G8OLgoI9QR7XWAjgOYC+Dzndtzqbpgl3Nq6g=","Reply-To":"kieran.bingham@ideasonboard.com","To":"=?utf-8?q?Niklas_S=C3=B6derlund?= ,\n\tlibcamera-devel@lists.libcamera.org","References":"<20190429191729.29697-1-niklas.soderlund@ragnatech.se>\n\t<20190429191729.29697-9-niklas.soderlund@ragnatech.se>","From":"Kieran Bingham ","Openpgp":"preference=signencrypt","Autocrypt":"addr=kieran.bingham@ideasonboard.com; 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Linux x86_64; rv:60.0) Gecko/20100101\n\tThunderbird/60.6.1","MIME-Version":"1.0","In-Reply-To":"<20190429191729.29697-9-niklas.soderlund@ragnatech.se>","Content-Type":"text/plain; charset=utf-8","Content-Language":"en-GB","Content-Transfer-Encoding":"8bit","Subject":"Re: [libcamera-devel] [PATCH 08/11] libcamera: media_device: Add\n\tfunctions to lock device for other processes","X-BeenThere":"libcamera-devel@lists.libcamera.org","X-Mailman-Version":"2.1.23","Precedence":"list","List-Id":"","List-Unsubscribe":",\n\t","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n\t","X-List-Received-Date":"Tue, 30 Apr 2019 07:55:12 -0000"}}]