linux-kernel - Re: [Intel-gfx] [PATCH v9 5/7] vfio: Define vfio based dma-buf operations

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Date:   Fri, 23 Jun 2017 11:15:54 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     "Zhang, Tina" <tina.zhang@...el.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Chen, Xiaoguang" <xiaoguang.chen@...el.com>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "Lv, Zhiyuan" <zhiyuan.lv@...el.com>,
        "Wang, Zhi A" <zhi.a.wang@...el.com>,
        "Wang, Zhenyu Z" <zhenyu.z.wang@...el.com>
Subject: Re: [Intel-gfx] [PATCH v9 5/7] vfio: Define vfio based dma-buf
 operations

On Fri, 23 Jun 2017 09:26:59 +0200
Gerd Hoffmann <kraxel@...hat.com> wrote:

>   Hi,
> 
> > Is this only going to support accelerated driver output, not basic
> > VGA
> > modes for BIOS interaction?  
> 
> Right now there is no vgabios or uefi support for the vgpu.
> 
> But even with that in place there still is the problem that the display
> device initialization happens before the guest runs and therefore there
> isn't an plane yet ...
> 
> > > Right now the experimental intel patches throw errors in case no
> > > plane
> > > exists (yet).  Maybe we should have a "bool is_enabled" field in
> > > the
> > > plane_info struct, so drivers can use that to signal whenever the
> > > guest
> > > has programmed a valid video mode or not (likewise for the cursor,
> > > which doesn't exist with fbcon, only when running xorg).  With that
> > > in
> > > place using the QUERY_PLANE ioctl also for probing looks
> > > reasonable.  
> > 
> > Sure, or -ENOTTY for ioctl not implemented vs -EINVAL for no
> > available
> > plane, but then that might not help the user know how a plane would
> > be
> > available if it were available.  
> 
> So maybe a "enum plane_state" (instead of "bool is_enabled")?  So we
> can clearly disturgish ENABLED, DISABLED, NOT_SUPPORTED cases?

What's the difference between NOT_SUPPORTED and -ENOTTY on the ioctl?
Perhaps a bit in a flags field could specify EN/DIS-ABLED and leave
room for things we're forgetting.  Keep in mind that we need to use
explicit width fields for a uapi structure, so __u32 vs enum.  Thanks,

Alex