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Mercier" , Julian Orth Cc: corbet@lwn.net, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, Sumit Semwal References: Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR2P281CA0081.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:9a::8) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|MW4PR12MB6949:EE_ X-MS-Office365-Filtering-Correlation-Id: 648388ac-2848-439f-f835-08deac2cca35 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016|22082099003|18002099003|56012099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:PH7PR12MB5685.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(376014)(1800799024)(366016)(22082099003)(18002099003)(56012099003);DIR:OUT;SFP:1101; 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Mercier wrote: > On Tue, May 5, 2026 at 6:00 AM Julian Orth wrote: >> >> On Tue, May 5, 2026 at 2:41 PM Christian König wrote: >>> >>> Hi Julian, >>> >>> On 5/5/26 14:25, Julian Orth wrote: >>>> In ab4c3dcf9a71582503b4fb25aeab884c696cab25 ("dma-buf: Remove DMA-BUF >>>> sysfs stats") the /sys/kernel/dmabuf/buffer directory was removed. >>>> >>>> I've been using this interface, specifically the exporter_name file, >>>> to detect dmabufs created via udmabuf. Such dmabufs show "udmabuf" in >>>> exporter_name. I've been doing this for two reasons: 1) to detect that >>>> mmap on such buffers will be fast and 2) to detect that GPU access to >>>> such buffers will be slow. >>> >>> Crap, I really hoped that Android was the only user of that sysfs interface since that approach turned out to be quite broken. >>> >>> It's number one rule on Linux that we don't break userspace. So I hope that you don't insist on bringing that interface back, but if you do I will just revert the removal until we found a better solution. >> >> Bringing it back shouldn't be necessary. >> >>> >>>> With the removal of that file, that detection mechanism no longer works. >>>> >>>> I'm not particularly fond of that mechanism but it was the only one >>>> providing that functionality that I could find at the time. If there >>>> is another one, ideally an ioctl on the dmabuf, please let me know. >>> >>> The virtual fdinfo file you can find under /proc/$pid/fdinfo/$fd also contains the exporter name for the DMA-buf. >>> >>> You can find the full documentation here: https://docs.kernel.org/filesystems/proc.html#dma-buffer-files >>> >>> Is that sufficient? >> >> I think that is sufficient. I probably didn't use fdinfo initially >> because 1) it's a lot more work to parse and 2) I wasn't sure if it >> was intended to be machine-readable or if there could sometimes be >> newlines in the values and such. >> >>> >>> Additional to that the debugfs for DMA-buf also contains that information and I'm open to the suggestion with the IOCTL. >> >> My application runs as a regular user so it cannot access /sys/kernel/debug. >> >> Having an IOCTL would be ideal if it is not too much work. I'll fall >> back to fdinfo for now. >> >> Thanks, Julian > > Phew, I'm glad fdinfo suits your needs. Yeah, exactly my thinking as well :) A college questioned me this week how to find DMA-buf stats for debugging and it turned out that Google points to the outdated DMA-buf sysfs documentation instead of the debugfs one. No idea why, maybe we need to improve the documentation here a bit. > Adding an ioctl would introduce new UAPI so I think we'd want to avoid > that unless absolutely necessary. CRIU has some similar requirements, e.g. they need to know the exporting driver of a DMA-buf. Not sure if the fdinfo file will be sufficient for that case or not. But yeah I agree that we only need this if actually necessary. Regards, Christian. > > Thanks, > T.J. > >>> >>> Regards, >>> Christian. >>> >>>> >>>> Shipping an entire BPF compiler in my application, which the original >>>> patch suggests as the replacement, is not an option when the removed >>>> alternative was simply reading a file. >>>> >>>> Thanks, Julian >>>