On Wed, May 03, 2017 at 09:25:17PM +0100, Chris Wilson wrote:
> Since kmap allows us to block we can pin the pages and use our normal
> page lookup routine making the implementation simple, or as some might
> say quick and dirty.
>
> Testcase: igt/drv_selftest/dmabuf
> Testcase: igt/prime_rw
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_gem_dmabuf.c | 24 ++++++
> drivers/gpu/drm/i915/selftests/i915_gem_dmabuf.c | 104 +++++++++++++++++++++++
> 2 files changed, 128 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/i915_gem_dmabuf.c
> index f225bf680b6d..6176e589cf09 100644
> --- a/drivers/gpu/drm/i915/i915_gem_dmabuf.c
> +++ b/drivers/gpu/drm/i915/i915_gem_dmabuf.c
> @@ -122,12 +122,36 @@ static void i915_gem_dmabuf_kunmap_atomic(struct dma_buf *dma_buf, unsigned long
> }
> static void *i915_gem_dmabuf_kmap(struct dma_buf *dma_buf, unsigned long page_num)
> {
> + struct drm_i915_gem_object *obj = dma_buf_to_obj(dma_buf);
> + struct page *page;
> +
> + if (page_num >= obj->base.size >> PAGE_SHIFT)
> + return NULL;
> +
> + if (!i915_gem_object_has_struct_page(obj))
> + return NULL;
> +
> + if (i915_gem_object_pin_pages(obj))
> + return NULL;
> +
> + /* Synchronisation is left to the caller (via .begin_cpu_access()) */
> + page = i915_gem_object_get_page(obj, page_num);
Looking at this again, we can call unpin_pages here and rely on the
page->mapcount to keep the struct page alive until it is unmapped.
As we exclude all objects not backed by struct_page, that should give us
the reassurance that we won't reuse the backing storage ourselves.
Though I am thinking about what assurances dmabuf should give that it
does not disappear whilst it is mapped.
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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