From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from galahad.ideasonboard.com ([185.26.127.97]:38778 "EHLO galahad.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756620AbcGIT3D (ORCPT ); Sat, 9 Jul 2016 15:29:03 -0400 From: Laurent Pinchart To: Sakari Ailus Cc: linux-media@vger.kernel.org, hverkuil@xs4all.nl, mchehab@osg.samsung.com Subject: Re: [PATCH v2.1 3/5] media: Refactor copying IOCTL arguments from and to user space Date: Sat, 09 Jul 2016 22:29:03 +0300 Message-ID: <3214203.0Eb5nWCm1s@avalon> In-Reply-To: <57308DAA.1000404@linux.intel.com> References: <1462360855-23354-4-git-send-email-sakari.ailus@linux.intel.com> <2507022.47E6MxOJNv@avalon> <57308DAA.1000404@linux.intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: Hi Sakari, On Monday 09 May 2016 16:16:26 Sakari Ailus wrote: > Laurent Pinchart wrote: > > On Wednesday 04 May 2016 16:09:51 Sakari Ailus wrote: > >> Refactor copying the IOCTL argument structs from the user space and back, > >> in order to reduce code copied around and make the implementation more > >> robust. > >> > >> As a result, the copying is done while not holding the graph mutex. > >> > >> Signed-off-by: Sakari Ailus > >> --- > >> since v2: > >> > >> - Remove function to calculate maximum argument size, replace by a char > >> array of 256 or kmalloc() if that's too small. > >> > >> drivers/media/media-device.c | 194 ++++++++++++++++--------------------- > >> 1 file changed, 94 insertions(+), 100 deletions(-) > >> > >> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c > >> index 9b5a88d..0797e4b 100644 > >> --- a/drivers/media/media-device.c > >> +++ b/drivers/media/media-device.c [snip] > >> @@ -453,10 +432,24 @@ static long __media_device_ioctl( > >> > >> info = &info_array[_IOC_NR(cmd)]; > >> > >> + if (_IOC_SIZE(info->cmd) > sizeof(__karg)) { > >> + karg = kmalloc(_IOC_SIZE(info->cmd), GFP_KERNEL); > >> + if (!karg) > >> + return -ENOMEM; > >> + } > >> + > >> + info->arg_from_user(karg, arg, cmd); > >> + > >> mutex_lock(&dev->graph_mutex); > >> - ret = info->fn(dev, arg); > >> + ret = info->fn(dev, karg); > >> mutex_unlock(&dev->graph_mutex); > >> > >> + if (!ret) > > > > How about if (!ret && info->arg_to_user) instead, and getting rid of > > copy_arg_to_user_nop() ? > > I thought of that, but I decided to optimise the common case --- which > is that the argument is copied back and forth. Not copying the argument > back is a very special case, we use it for a single compat IOCTL. > > That said, we could use it for the proper ENUM_LINKS as well. Still that > does not change what's normal. We're talking about one comparison and one branching instruction (that will not be taken in the common case). Is that micro-optimization really worth it in an ioctl path that is not that performance-critical ? If you think it is, could you analyse what the impact of the copy_arg_to_user_nop() function on cache locality is for the common case ? ;-) -- Regards, Laurent Pinchart