Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757215AbcCCRSt (ORCPT ); Thu, 3 Mar 2016 12:18:49 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:47561 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755254AbcCCRSr (ORCPT ); Thu, 3 Mar 2016 12:18:47 -0500 Date: Thu, 3 Mar 2016 09:18:46 -0800 From: Greg Kroah-Hartman To: "Ning, Yu" Cc: Jin Qian , "linux-kernel@vger.kernel.org" , Jeff Sharkey , David Turner , "pprabhu@google.com" Subject: Re: allocate an official device major number for virtio device? Message-ID: <20160303171846.GC20826@kroah.com> References: <20160303012530.GA23521@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1610 Lines: 47 A: No. Q: Should I include quotations after my reply? http://daringfireball.net/2007/07/on_top On Thu, Mar 03, 2016 at 03:52:20AM +0000, Ning, Yu wrote: > Well, virtio_blk does use dynamic major number allocation, but the > allocated block major just happens to fall in the "experimental" range > (240-254)... That all depends on what else is registered in the system at the moment. > In more detail: > > virtio_blk calls register_blkdev() with major = 0 in init() (drivers/block/virtio_blk.c:872): > > major = register_blkdev(0, "virtblk"); > > This line has been there since day one. And register_blkdev() implements dynamic major allocation pretty straightforwardly: > > /* temporary */ > if (major == 0) { > for (index = ARRAY_SIZE(major_names)-1; index > 0; index--) { > if (major_names[index] == NULL) > break; > } > > So it goes from index = 254 to 1 and picks the first unused. > Apparently, there's a good chance that the allocated major is between > 240-254 (although lower numbers are also possible, theoretically). > Indeed, we always get 253 for virtio_blk with the x86_64 Android > emulator kernel. > > But "dynamic" means we can't rely on checking major == 253 to detect > virtio_blk. Nor should you, why would you care? > That's why we are doing a fnmatch() using pattern > /sys/devices/*/block/vd* instead. Is that the recommended approach? Yes, or just look at the device node that is already created in /dev/ for you automatially by devtmpfs. Doesn't that work as expected today? I still don't understand the issue you are having here at all, sorry. greg k-h