linux-kernel - Re: [PATCH 4/8] drm/ttm: factor out ttm_bo_mmap_vma

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Message-ID: <20190917083425.kwwqyn463gn3mghf@sirius.home.kraxel.org>
Date:   Tue, 17 Sep 2019 10:34:25 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>,
        Huang Rui <ray.huang@....com>,
        Christian Koenig <christian.koenig@....com>
Subject: Re: [PATCH 4/8] drm/ttm: factor out ttm_bo_mmap_vma_setup

On Fri, Sep 13, 2019 at 03:05:34PM +0200, Thomas Zimmermann wrote:

> > +void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_struct *vma)
> > +{
> > +	vma->vm_ops = &ttm_bo_vm_ops;
> > +
> > +	/*
> > +	 * Note: We're transferring the bo reference to
> > +	 * vma->vm_private_data here.
> > +	 */
> > +
> > +	vma->vm_private_data = bo;
> > +
> > +	/*
> > +	 * We'd like to use VM_PFNMAP on shared mappings, where
> > +	 * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
> > +	 * but for some reason VM_PFNMAP + x86 PAT + write-combine is very
> > +	 * bad for performance. Until that has been sorted out, use
> > +	 * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
> > +	 */
> > +	vma->vm_flags |= VM_MIXEDMAP;
> > +	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
> > +}
> > +EXPORT_SYMBOL(ttm_bo_mmap_vma_setup);
> 
> To me, this function looks like an internal helper that should rather
> remain internal.

Well, I'm moving that to a helper exactly to avoid drm gem ttm helpers
messing with ttm internals.  To not them initialize vm_flags for
example, and to avoid exporting ttm_bo_vm_ops.  Also to make sure ttm bo
vma's are initialized the same way no matter which code path was taken
to mmap the object.

> As mentioned in my reply to patch 5, maybe re-using
> ttm_fbdev_mmap() could help.

No, the check in that function prevents that from working.

cheers,
  Gerd