Re: [PATCH v3] intel: New libdrm interface to create unbound wc user mappings for objects

[Damien Lespiau <damien.lespiau@intel.com>]

On Tue, Oct 28, 2014 at 06:39:27PM +0530, akash.goel@intel.com wrote:
> From: Akash Goel <akash.goel@intel.com>
> 
> A new libdrm interface 'drm_intel_gem_bo_map_wc' is provided by this
> patch. Through this interface Gfx clients can create write combining
> virtual mappings of the Gem object. It will provide the same funtionality
> of 'mmap_gtt' interface without the constraints of limited aperture space,
> but provided clients handles the linear to tile conversion on their own.
> This patch is intended for improving the CPU write operation performance,
> as with such mapping, writes are almost 50% faster than with mmap_gtt.
> Also it avoids the Cache flush after update from CPU side, when object is
> passed on to GPU, which will be the case if regular mmap interface is used.
> This type of mapping is specially useful in case of sub-region
> update, i.e. when only a portion of the object is to be updated.
> Also there is a support for the unsynchronized version of this interface
> named 'drm_intel_gem_bo_map_wc_unsynchronized', through which wc virtual
> mappings, but unsynchronized one, can be created of the Gem object.
> To ensure the cache coherency, before using this mapping, the GTT domain has
> been reused here. This provides the required Cache flush if the object is in
> CPU domain or synchronization against the concurrent rendering
> 
> The drm_i915_gem_mmap structure (for the DRM_I915_GEM_MMAP_IOCTL) has been
> extended with a new flags field (defaulting to 0 for existent users). In
> order for userspace to detect the extended ioctl, a new parameter
> I915_PARAM_HAS_EXT_MMAP has been added for versioning the ioctl interface.
> 
> v2: Aligned with the v2 of the corresponding kernel patch (Chris)
> v3: Added the unmap calls for the wc mapping (Damien)
>     Added the param feature check before creating the wc mapping & reduced
>     the vma limit (Chris)
> 
> Signed-off-by: Akash Goel <akash.goel@intel.com>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

I'm still not super convinced by the full copy and paste of the function
where all I can see changing is the pointer storing the virtual address
and the flags. Something I'm missing? What do others think (I'm not
the maintainer of anything in libdrm).

-- 
Damien
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