Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 04/18/2018 01:55 PM, Roger Pau Monné wrote:
> On Wed, Apr 18, 2018 at 01:39:35PM +0300, Oleksandr Andrushchenko wrote:
>> On 04/18/2018 01:18 PM, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: Xen-devel [mailto:xen-devel-bounces@lists.xenproject.org] On Behalf
>>>> Of Roger Pau Monné
>>>> Sent: 18 April 2018 11:11
>>>> To: Oleksandr Andrushchenko <andr2000@gmail.com>
>>>> Cc: jgross@suse.com; Artem Mygaiev <Artem_Mygaiev@epam.com>;
>>>> Dongwon Kim <dongwon.kim@intel.com>; airlied@linux.ie;
>>>> Oleksandr_Andrushchenko@epam.com; linux-kernel@vger.kernel.org; dri-
>>>> devel@lists.freedesktop.org; Potrola, MateuszX
>>>> <mateuszx.potrola@intel.com>; xen-devel@lists.xenproject.org;
>>>> daniel.vetter@intel.com; boris.ostrovsky@oracle.com; Matt Roper
>>>> <matthew.d.roper@intel.com>
>>>> Subject: Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy
>>>> helper DRM driver
>>>>
>>>> On Wed, Apr 18, 2018 at 11:01:12AM +0300, Oleksandr Andrushchenko
>>>> wrote:
>>>>> On 04/18/2018 10:35 AM, Roger Pau Monné wrote:
>>>> After speaking with Oleksandr on IRC, I think the main usage of the
>>>> gntdev extension is to:
>>>>
>>>> 1. Create a dma-buf from a set of grant references.
>>>> 2. Share dma-buf and get a list of grant references.
>>>>
>>>> I think this set of operations could be broken into:
>>>>
>>>> 1.1 Map grant references into user-space using the gntdev.
>>>> 1.2 Create a dma-buf out of a set of user-space virtual addresses.
>>>>
>>>> 2.1 Map a dma-buf into user-space.
>>>> 2.2 Get grefs out of the user-space addresses where the dma-buf is
>>>>       mapped.
>>>>
>>>> So it seems like what's actually missing is a way to:
>>>>
>>>>    - Create a dma-buf from a list of user-space virtual addresses.
>>>>    - Allow to map a dma-buf into user-space, so it can then be used with
>>>>      the gntdev.
>>>>
>>>> I think this is generic enough that it could be implemented by a
>>>> device not tied to Xen. AFAICT the hyper_dma guys also wanted
>>>> something similar to this.
>> Ok, so just to summarize, xen-zcopy/hyper-dmabuf as they are now,
>> are no go from your POV?
> My opinion is that there seems to be a more generic way to implement
> this, and thus I would prefer that one.
>
>> Instead, we have to make all that fancy stuff
>> with VAs <-> device-X and have that device-X driver live out of drivers/xen
>> as it is not a Xen specific driver?
> That would be my preference if feasible, simply because it can be
> reused by other use-cases that need to create dma-bufs in user-space.
There is a use-case I have: a display unit on my target has a DMA
controller which can't do scatter-gather, e.g. it only expects a
single starting address of the buffer.
In order to create a dma-buf from grefs in this case
I allocate memory with dma_alloc_xxx and then balloon pages of the
buffer and finally map grefs onto this DMA buffer.
This way I can give this shared buffer to the display unit as its bus
addresses are contiguous.

With the proposed solution (gntdev + device-X) I won't be able to 
achieve this,
as I have no control over from where gntdev/balloon drivers get the pages
(even more, those can easily be out of DMA address space of the display 
unit).

Thus, even if implemented, I can't use this approach.
>
> In any case I just knew about dma-bufs this morning, there might be
> things that I'm missing.
>
> Roger.