[PATCH v6 09/11] modesetting: Blacklist UDL from PRIME sync

[Hans de Goede]

Hi,

On 11-06-16 02:21, Alex Goins wrote:
> UDL (USB 2.0 DisplayLink DRM driver) has strange semantics when it comes to
> vblank events and damage tracking when page flipping.
>
> When doing a page flip, it will instantly raise a vblank event without
> waiting for vblank. However, it has no support for DRM_IOCTL_WAIT_VBLANK.
> It also seems to have some issues with damage tracking when page flipping.
>
> It's possible to get something semi-working by hacking around these issues,
> but even then there isn't much value-add vs single buffered PRIME, and it
> reduces maintainability and adds additional risks to the modesetting driver
> when running with more well-behaved DRM drivers.
>
> Work needs to be done on UDL in order to properly support synchronized
> PRIME. For now, just blacklist it, causing RandR to fall back to
> unsynchronized PRIME.

Hmm, I've been working on getting a driver for another video-output device
using usb transport (mostly to teach myself kms), see:

http://www.spinics.net/lists/dri-devel/msg109311.html

This has some of the same limitations as the udl driver, for one it
raise a vblank event without waiting for vblank. I can probably fix this to
at least wait till all data has been send over the usb-bus, but there
is no way I can get actual vblank info on this device.

As such this device likely also needs to be excluded from using
synchronized prime, which makes me think that maybe we just need
to blacklist synchronized prime for any devices using a usb transport
instead of blacklisting UDL ?

Regards,

Hans


p.s.

Since you've been working on prime and the modesetting driver, I've
a set of pending prime / modesetting related patches which need a
review, and which may interest you:

https://patchwork.freedesktop.org/patch/89722/
https://patchwork.freedesktop.org/patch/90777/
https://patchwork.freedesktop.org/patch/90982/
https://patchwork.freedesktop.org/patch/90778/
https://patchwork.freedesktop.org/patch/90779/





>
> v1: N/A
> v2: N/A
> v3: Initial commit
> v4: Move check to driver.c for consistency/visibility
> v5: Refactor to accomodate earlier changes
> v6: Rebase onto ToT
>
> Signed-off-by: Alex Goins <agoins at nvidia.com>
> ---
>  hw/xfree86/drivers/modesetting/driver.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/hw/xfree86/drivers/modesetting/driver.c b/hw/xfree86/drivers/modesetting/driver.c
> index ab3b028..f7abd1e 100644
> --- a/hw/xfree86/drivers/modesetting/driver.c
> +++ b/hw/xfree86/drivers/modesetting/driver.c
> @@ -974,6 +974,7 @@ msEnableSharedPixmapFlipping(RRCrtcPtr crtc, PixmapPtr front, PixmapPtr back)
>      ScrnInfoPtr scrn = xf86ScreenToScrn(screen);
>      modesettingPtr ms = modesettingPTR(scrn);
>      xf86CrtcPtr xf86Crtc = crtc->devPrivate;
> +    drmVersionPtr version;
>
>      if (!xf86Crtc)
>          return FALSE;
> @@ -986,6 +987,12 @@ msEnableSharedPixmapFlipping(RRCrtcPtr crtc, PixmapPtr front, PixmapPtr back)
>      if (ms->drmmode.reverse_prime_offload_mode)
>          return FALSE;
>
> +    version = drmGetVersion(ms->drmmode.fd);
> +
> +    /* UDL has several issues that prevent double buffering */
> +    if (!strncmp("udl", version->name, version->name_len))
> +        return FALSE;
> +
>      return drmmode_EnableSharedPixmapFlipping(xf86Crtc, &ms->drmmode,
>                                                front, back);
>  }
>