[PATCH] modesetting: Fix X crash in ms_dirty_update()

[Michel Dänzer]

On 2018-08-03 03:27 PM, Jim Qu wrote:
> On some Intel iGPU + AMD dGPU platform, when connect extern display
> from dGPU, X will crash, show the log like:
> 
> randr: falling back to unsynchronized pixmap sharing
> (EE)
> (EE) Backtrace:
> (EE) 0: /usr/lib/xorg/Xorg (xorg_backtrace+0x4e)
> (EE) 1: /usr/lib/xorg/Xorg (0x55cb0151a000+0x1b5ce9)
> (EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f1587a1d000+0x11390)
> (EE)
> (EE) Segmentation fault at address 0x0
> (EE)
> 
> There is NULL pointer accessing on ent->slave_dst->drawable.pScreen->
> SharedPixmapNotifyDamage.
> 
> On the platform, since the dGPU is GPU device, so that the iGPU is
> output master device. SharedPixmapNotifyDamage() should be called when
> current device is output slave.
> 
> Change-Id: Id633e29c126670ee64ff1f5f79d489e5068cd439
> Signed-off-by: Jim Qu <Jim.Qu at amd.com>
> ---
>  hw/xfree86/drivers/modesetting/driver.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/xfree86/drivers/modesetting/driver.c b/hw/xfree86/drivers/modesetting/driver.c
> index 9362370..6022315 100644
> --- a/hw/xfree86/drivers/modesetting/driver.c
> +++ b/hw/xfree86/drivers/modesetting/driver.c
> @@ -640,20 +640,21 @@ ms_dirty_update(ScreenPtr screen, int *timeout)
>      xorg_list_for_each_entry(ent, &screen->pixmap_dirty_list, ent) {
>          region = DamageRegion(ent->damage);
>          if (RegionNotEmpty(region)) {
> -            msPixmapPrivPtr ppriv =
> -                msGetPixmapPriv(&ms->drmmode, ent->slave_dst);
> +            if (screen->isGPU) {
> +                msPixmapPrivPtr ppriv =
> +                    msGetPixmapPriv(&ms->drmmode, ent->slave_dst);
>  
> -            if (ppriv->notify_on_damage) {
> -                ppriv->notify_on_damage = FALSE;
> +                if (ppriv->notify_on_damage) {
> +                    ppriv->notify_on_damage = FALSE;
>  
> -                ent->slave_dst->drawable.pScreen->
> -                    SharedPixmapNotifyDamage(ent->slave_dst);
> -            }
> -
> -            /* Requested manual updating */
> -            if (ppriv->defer_dirty_update)
> -                continue;
> +                    ent->slave_dst->drawable.pScreen->
> +                        SharedPixmapNotifyDamage(ent->slave_dst);
> +                }
>  
> +                /* Requested manual updating */
> +                if (ppriv->defer_dirty_update)
> +                    continue;
> +            }

I don't think this is right. E.g. why would the slave driver call its
own SharedPixmapNotifyDamage hook? Also, ppriv->defer_dirty_update is
only set to TRUE by hooks which are only called for the master screen.

So, I think the start of the new block needs to be something like:

            if (!screen->isGPU) {
                msPixmapPrivPtr ppriv =
                    msGetPixmapPriv(&ms->drmmode,
ent->slave_dst->master_pixmap);

and msStartFlippingPixmapTracking and msStopFlippingPixmapTracking also
need to use ->master_pixmap instead of the slave pixmaps.


Not sure about the defer_dirty_update related code, that might only make
sense in the slave.


The more I look at this PRIME synchronization related code, the more I
wonder if it was ever tested with master and slave screens using
different drivers...


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer