[Intel-gfx] [PATCH] drm/i915: Document that mmap forwarding is discouraged

[Cheng, Yao]

Nice :) thanks for this!

> -----Original Message-----
> From: David Herrmann [mailto:dh.herrmann at gmail.com]
> Sent: Thursday, October 16, 2014 8:03 PM
> To: Daniel Vetter
> Cc: Intel Graphics Development; Cheng, Yao
> Subject: Re: [PATCH] drm/i915: Document that mmap forwarding is
> discouraged
> 
> Hi
> 
> On Thu, Oct 16, 2014 at 12:28 PM, Daniel Vetter <daniel.vetter at ffwll.ch>
> wrote:
> > Too many new drm driver writers seem to look at i915 for inspiration.
> > But we have two ways to do mmap, so discourage readers from the old,
> > ugly version. In a new driver we'd just expose two mmap offsets per
> > object, one for the gtt map and the other for the cpu map.
> 
> Yes, please! (also fine with Jani's concerns)
> 
> Reviewed-by: David Herrmann <dh.herrmann at gmail.com>
> 
> Thanks
> David
> 
> > Cc: "Cheng, Yao" <yao.cheng at intel.com>
> > Cc: David Herrmann <dh.herrmann at gmail.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c
> > b/drivers/gpu/drm/i915/i915_gem.c index e9c783d55612..09d859b89aac
> > 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -1466,6 +1466,15 @@ unlock:
> >   *
> >   * While the mapping holds a reference on the contents of the object, it
> doesn't
> >   * imply a ref on the object itself.
> > + *
> > + * IMPORTANT:
> > + *
> > + * DRM driver writers who look a this function as an example for how
> > + to do GEM
> > + * mmap support, please don't implement mmap support like here. The
> > + modern way
> > + * to implement DRM mmap support is with an mmap offset ioctl (like
> > + * i915_gem_mmap_gtt) and then using the mmap syscall on the DRM fd
> directly.
> > + * That way debug tooling like valgrind will understand what's going
> > + on, hiding
> > + * the mmap call in a driver private ioctl will break that.
> >   */
> >  int
> >  i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
> > --
> > 1.9.3
> >