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

[Jani Nikula]

On Thu, 16 Oct 2014, 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.
>
> 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.

Maybe a sentence here about why we're doing things differently, instead
of merely "do as I say, don't do as I do"? It smells like hysterical
raisins, but no harm in spelling it out, right?

BR,
Jani.

>   */
>  int
>  i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
> -- 
> 1.9.3
>
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-- 
Jani Nikula, Intel Open Source Technology Center