[PATCH 00/15] Share TTM code among framebuffer drivers

[Dave Airlie]

On Tue, 9 Apr 2019 at 17:12, kraxel at redhat.com <kraxel at redhat.com> wrote:
>
>   Hi,
>
> > If not for TTM, what would be the alternative? One VMA manager per
> > memory region per device?
>
> Depends pretty much on the device.
>
> The cirrus is a display device with only 4 MB of vram.  You can't fit
> much in there.  A single 1024x768 @ 24bpp framebuffer needs more 50%
> of the video memory already.  Which is why the cirrus driver (before the
> rewrite) had to migrate buffers from/to vram on every page flip[1].  Which
> is one[2] of the reasons why cirrus (after rewrite) doesn't ttm-manage the
> vram any more.  gem objects are managed with the shmem helpers instead
> and the active framebuffer is blitted to vram.
>
> The qemu stdvga (bochs driver) has 16 MB vram by default and can be
> configured to have up to 256 MB.  Plenty of room even for multiple 4k
> framebuffers if needed.  So for the bochs driver all the ttm bo
> migration logic is not needed, it could just store everything in vram.

To clarify I assume you mean it doesn't need the migrate each bo
logic, but it still needs the when VRAM fills up migrate stuff logic.

Dave.