linux-kernel - Re: [PATCH v2 15/15] drm/bochs: reserve bo for pin/unpin

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Date:   Wed, 9 Jan 2019 21:51:58 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...hat.com>,
        Oleksandr Andrushchenko <andr2000@...il.com>,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR BOCHS VIRTUAL GPU" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 15/15] drm/bochs: reserve bo for pin/unpin

  Hi,

> > If I understand things correctly it is valid to set all import/export
> > callbacks (prime_handle_to_fd, prime_fd_to_handle,
> > gem_prime_get_sg_table, gem_prime_import_sg_table) to NULL when not
> > supporting dma-buf import/export and still advertise DRIVER_PRIME to
> > indicate the other prime callbacks are supported (so generic fbdev
> > emulation can use gem_prime_vmap etc).  Is that correct?
> 
> I'm not sure how much that's a good idea ... Never thought about it
> tbh. All the fbdev/dma-buf stuff has plenty of hacks and
> inconsistencies still, so I guess we can't make it much worse really.

Setting prime_handle_to_fd + prime_fd_to_handle to NULL has the effect
that drm stops advertising DRM_PRIME_CAP_{IMPORT,EXPORT} to userspace.

Which looks better to me than telling userspace we support it then throw
errors unconditionally when userspace tries to use that.

> > Is it possible to export TTM_PL_VRAM objects (with backing storage being
> > a pci memory bar)?  If so, how?
> 
> Not really in general. amdgpu upcasts to amdgpu_bo (if it's amgpu BO)
> and then knows the internals so it can do a proper pci peer2peer
> mapping. Or at least there's been lots of patches floating around to
> make that happen.

That is limited to bo sharing between two amdgpu devices, correct?

> I think other drivers migrate the bo out of VRAM.

Well, that doesn't look too useful.  bochs and qxl virtual hardware
can't access buffers outside VRAM.  So, while I could migrate the
buffers to RAM (via memcpy) when exporting they would at the same time
become unusable for the GPU ...

> > On importing:
> >
> > Importing into TTM_PL_TT object looks easy again, at least when the
> > object is actually stored in RAM.  What if not?
> 
> They are all supposed to be stored in RAM. Note that all current ttm
> importers totally break the abstraction, by taking the sg list,
> throwing the dma mapping away and assuming there's a struct page
> backing it. Would be good if we could stop spreading that abuse - the
> dma-buf interfaces have been modelled after the ttm bo interfaces, so
> shouldn't be too hard to wire this up correctly.

Ok.  With virtio-gpu (where objects are backed by RAM pages anyway)
wiring this up should be easy.

But given there is no correct sample code I can look at it would be cool
if you could give some more hints how this is supposed to work.  The
gem_prime_import_sg_table() callback gets a sg list passed in after all,
so I probably would have tried to take the sg list too ...

> > Importing into TTM_PL_VRAM:  Impossible I think, without copying over
> > the data.  Should that be done?  If so, how?  Or is it better to just
> > not support import then?
> 
> Hm, since you ask about TTM concepts and not what this means in terms
> of dma-buf:

Ok, more details on the quesion:

dma-buf: whatever the driver gets passed into the
gem_prime_import_sg_table() callback.

import into TTM_PL_VRAM: qemu driver which supports VRAM storage only
(bochs, qxl), so the buffer has to be stored there if we want do
something with it (like scanning out to a crtc).

> As long as you upcast to the ttm_bo you can do whatever
> you want to really.

Well, if the dma-buf comes from another device (say export vgem bo, then
try import into bochs/qxl/virtio) I can't upcast.

When the dma-buf comes from the same device drm_gem_prime_import_dev()
will notice and take a shortcut (skip import, just increase refcount
instead), so I don't have to worry about that case in the
gem_prime_import_sg_table() callback.

> But with plain dma-buf this doesn't work right now
> (least because ttm assumes it gets system RAM on import, in theory you
> could put the peer2peer dma mapping into the sg list and it should
> work).

Well, qemu display devices don't have peer2peer dma support.
So I guess the answer is "doesn't work".

cheers,
  Gerd