[PATCH] drm/cma: correctly handle non-zero offset for mmap

[Daniel Vetter]

On Wed, Sep 27, 2017 at 4:01 PM, Steven Price <Steven.Price at arm.com> wrote:
> From: Tu Vuong <tu.vuong at arm.com>
>
> When a CMA GEM object is exported via DRM PRIME it should be possible
> to mmap the object using an offset. However drm_gem_cma_mmap_obj always
> zeroed vm_pgoff.

No, at least no one ever had run into this. Pretty much all drivers
work like this.

> Fix this by moving the zeroing of vm_pgoff to drm_gem_cma_mmap (which
> is only used for non-PRIME mmap) and correct the size parameter in the
> call to dma_mmap_wc as the offset may not be non-zero.

Since this is an uabi change we need the corresponding patch to
upstream open source userspace. Reviewed and all that and ready for
acceptance. See

https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#open-source-userspace-requirements

> Signed-off-by: Tu Vuong <tu.vuong at arm.com>
> Signed-off-by: Steven Price <steven.price at arm.com>
> Reviewed-by: Brian Starkey <brian.starkey at arm.com>
> CC: Brian Starkey <brian.starkey at arm.com>
> CC: Liviu Dudau <Liviu.Dudau at arm.com>

Thanks, Daniel

> ---
>  drivers/gpu/drm/drm_gem_cma_helper.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index 373e33f22be4..25828b33c5be 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -276,15 +276,12 @@ static int drm_gem_cma_mmap_obj(struct drm_gem_cma_object *cma_obj,
>         int ret;
>
>         /*
> -        * Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
> -        * vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
> -        * the whole buffer.
> +        * Clear the VM_PFNMAP flag that was set by drm_gem_mmap()
>          */
>         vma->vm_flags &= ~VM_PFNMAP;
> -       vma->vm_pgoff = 0;
>
>         ret = dma_mmap_wc(cma_obj->base.dev->dev, vma, cma_obj->vaddr,
> -                         cma_obj->paddr, vma->vm_end - vma->vm_start);
> +                         cma_obj->paddr, cma_obj->base.size);
>         if (ret)
>                 drm_gem_vm_close(vma);
>
> @@ -322,6 +319,12 @@ int drm_gem_cma_mmap(struct file *filp, struct vm_area_struct *vma)
>         gem_obj = vma->vm_private_data;
>         cma_obj = to_drm_gem_cma_obj(gem_obj);
>
> +       /*
> +        * Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
> +        * want to map the whole buffer.
> +        */
> +       vma->vm_pgoff = 0;
> +
>         return drm_gem_cma_mmap_obj(cma_obj, vma);
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_cma_mmap);
> --
> 2.11.0
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch