linux-kernel - Re: [RFC] dma-buf: Implement test module

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Message-ID: <20131214174225.GP9804@phenom.ffwll.local>
Date:	Sat, 14 Dec 2013 18:42:25 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linaro-mm-sig@...ts.linaro.org, dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] dma-buf: Implement test module

On Sat, Dec 14, 2013 at 01:16:21PM +0100, Thierry Reding wrote:
> On Thu, Dec 12, 2013 at 06:04:13PM -0800, Greg Kroah-Hartman wrote:
> > On Thu, Dec 12, 2013 at 03:36:29PM +0100, Thierry Reding wrote:
> > > This is a simple test module that can be used to allocate, export and
> > > delete DMA-BUF objects. It can be used to test DMA-BUF sharing in
> > > systems that lack a real second driver.
> > > 
> > > Signed-off-by: Thierry Reding <treding@...dia.com>
> > > ---
> > >  drivers/base/Kconfig        |   4 +
> > >  drivers/base/Makefile       |   1 +
> > >  drivers/base/dma-buf-test.c | 308 ++++++++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 313 insertions(+)
> > >  create mode 100644 drivers/base/dma-buf-test.c
> > > 
> > > diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> > > index e373671652b0..bed2abb9491b 100644
> > > --- a/drivers/base/Kconfig
> > > +++ b/drivers/base/Kconfig
> > > @@ -200,6 +200,10 @@ config DMA_SHARED_BUFFER
> > >  	  APIs extension; the file's descriptor can then be passed on to other
> > >  	  driver.
> > >  
> > > +config DMA_BUF_TEST
> > > +	tristate "DMA-BUF test module"
> > > +	depends on DMA_SHARED_BUFFER
> > 
> > We need some good documentation here.
> 
> I agree. The description should go into more details about what exactly
> this is meant to address.
> 
> > > > +static struct miscdevice dmabuf_device = {
> > > +	.minor = 128,
> > 
> > Why did you pick this minor?  Why not just make it dynamic?
> 
> It seemed like minors are statically allocated for miscdevice and 128
> seemed to be as good as any. The tentative plan was to go through the
> official way of having one allocated as explained in the comment in
> include/linux/miscdevice.h.
> 
> Reading that comment again, there's MISC_DYNAMIC_MINOR, which I guess
> would be appropriate here. Chances are that if you want to test DMA-BUF
> you'll have a reasonably modern userspace that will create the device
> dynamically.
> 
> Alternatively I guess I could instead turn this into a more full-fledged
> cdev and do the whole alloc_chrdev_region() dance. Although that will
> only allocate the major dynamically.
> 
> I'm not aware of any function that just allocates a single major/minor
> pair completely dynamically. Is there one that you could point me to?

miscdev seems appropriate for me, I don't think we'll ever need a 2nd
dma-buf node. After all the entire point of sharing is to have
system-wide access, so having a 2nd dma-buf domain/thing doesn't look
useful at all.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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