linux-kernel - Re: [RFC PATCH 2/2] drm/vgem: Enable dmabuf import interfaces

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Message-ID: <20170407073957.GT10496@nuc-i3427.alporthouse.com>
Date:   Fri, 7 Apr 2017 08:39:57 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [RFC PATCH 2/2] drm/vgem: Enable dmabuf import interfaces

On Thu, Apr 06, 2017 at 04:18:33PM -0700, Laura Abbott wrote:
> 
> Enable the GEM dma-buf import interfaces in addition to the export
> interfaces. This lets vgem be used as a test source for other allocators
> (e.g. Ion).
> 
> +int vgem_gem_get_pages(struct drm_vgem_gem_object *obj)
> +{
> +	struct page **pages;
> +
> +	if (obj->pages)
> +		return 0;
> +
> +	pages = drm_gem_get_pages(&obj->base);
> +	if (IS_ERR(pages)) {
> +		return PTR_ERR(pages);
> +	}
> +
> +	obj->pages = pages;

That's a significant loss in functionality (it requires the entire
object to fit into physical memory at the same time and requires a large
vmalloc for 32b systems), for what? vgem only has the ability to mmap
and export a fd -- both of which you already have if you have the dmabuf
fd. The only extra interface is the signaling, which does not yet have a
direct correspondence on dmabuf.

(An obvious way to keep both would be to move the get_pages to importing
and then differentiate in the faulthandler where to get the page from.)

Can you provide more details on how you are using vgem to justify the
changes? I'm also not particularly happy in losing testing of a virtual
platform device from our CI.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre