linux-kernel - Re: [PATCH 0/7] IB/hfi1: Remove write() and use ioctl() for user access

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Date:	Thu, 14 Apr 2016 12:56:59 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Dennis Dalessandro <dennis.dalessandro@...el.com>
Cc:	Ira Weiny <ira.weiny@...el.com>, dledford@...hat.com,
	linux-rdma@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] IB/hfi1: Remove write() and use ioctl() for user
 access

On Thu, Apr 14, 2016 at 02:42:01PM -0400, Dennis Dalessandro wrote:
> >It certainly can't be years.
> 
> Does fixing the current write()/writev() problem have any real
> impact on how we proceed for the "1 char dev to rule them all" idea?

We aren't going to take a bad uAPI into mainline. So how many times do
you want to redo the userspace? I have no objection to the patch
landing, just as long as it stays in staging until we have the uAPI
discussion as a community.

As for the 'one char device', I actually think it would be really
simple.

Add a new uverbs ioctl:

 int hfi1_fd = ioctl(uverbs_fd, RDMA_GET_DRIVER_OPS_FD, "psm2.intel.com");

 ioctl(hfi1_fd, HFI1_IOCTL_ASSIGN_CTXT, ...);
 write(hfi1_fd, ...);

At least that gives us far better options for discovery and versioning
of this stuff than a driver-specific char device.

[eg this would use anon_inode_getfile, like event fds, completion
channels, etc]

You guys need this the most, propose something already.

* driver specific ioctls might be nicer, but people argue that is not
  performant enough for what you want... Unclear to me.

Jason