linux-kernel - Re: [PATCH] debugfs: Add proxy function for the mmap file operation

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Date:   Thu, 1 Sep 2016 16:43:54 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Liviu Dudau <Liviu.Dudau@....com>
Cc:     Nicolai Stange <nicstange@...il.com>,
        Brian Starkey <brian.starkey@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] debugfs: Add proxy function for the mmap file operation

On Thu, Sep 01, 2016 at 01:50:39PM +0100, Liviu Dudau wrote:
> On Thu, Sep 01, 2016 at 08:19:33AM +0200, Greg Kroah-Hartman wrote:
> > On Wed, Aug 31, 2016 at 04:23:52PM +0100, Liviu Dudau wrote:
> > > On Wed, Aug 31, 2016 at 03:07:49PM +0200, Greg Kroah-Hartman wrote:
> > > > On Fri, Aug 05, 2016 at 01:11:45PM +0200, Nicolai Stange wrote:
> > > > > Brian Starkey <brian.starkey@....com> writes:
> > > > > 
> > > > > > On Tue, Aug 02, 2016 at 07:31:36PM +0200, Nicolai Stange wrote:
> > > > > >>Nicolai Stange <nicstange@...il.com> writes:
> > > > > >>> However, if you wish to have some mmapable debugfs file which *can* go
> > > > > >>> away, introducing mmap support in the debugfs full proxy is perfectly
> > > > > >>> valid. But please see below.
> > > > > >>
> > > > > >>Assuming that you've got such a use case, please consider resending your
> > > > > >>patch along with the Cocci script below (and the Coccinelle team CC'ed,
> > > > > >>of course). If OTOH your mmapable debugfs files are never removed, just
> > > > > >>drop this message and use debugfs_create_file_unsafe() instead.
> > > > > >
> > > > > > So we do have an implementation using this, but it's likely we will
> > > > > > keep it out-of-tree (it's a stop-gap until we can get a non-debugfs
> > > > > > implementation of the functionality into mainline).
> > > > > >
> > > > > > Do you think it's worth merging this (and your cocci script) anyway to
> > > > > > save someone else doing the same thing later?
> > > > > 
> > > > > I personally think that having ->mmap() support in debugfs would be a
> > > > > good thing to have in general and I expect there to be some further
> > > > > demand in the future.
> > > > 
> > > > Ugh, mmap in debugfs, that's funny.  And sad...
> > > 
> > > Yeah.
> > > 
> > > While our need for the mmap-ing the debugfs entry is at best a temporary
> > > option and a hack, I would be interested to know what alternatives could
> > > be used to read a large amount of data that does not need the seq_operations
> > > API? The out-of-tree proof-of-concept code that we have to interact with
> > > a memory write engine needs to be able to access the output buffer from
> > > userspace, but that output buffer is created by the kernel KMS driver.
> > 
> > What type of debugging do you need this for?
> 
> Taking snapshots of a composition scene using the KMS driver for Mali DP.

So it's not just debugging?  This is a "real" thing that code will rely
on?  If so, that's not good, don't ever use debugfs for that.

good luck!

greg k-h