[bug report] drm/xen-front: Add support for Xen PV display frontend

[Oleksandr Andrushchenko]

On 4/21/20 2:51 PM, Dan Carpenter wrote:
> It turns out there aren't that many of these in xen.
>
> $ grep IS_ERR_OR_NULL drivers/gpu/drm/xen/ -Rn
> drivers/gpu/drm/xen/xen_drm_front_kms.c:63:     if (IS_ERR_OR_NULL(fb))
> drivers/gpu/drm/xen/xen_drm_front_gem.c:86:     if (IS_ERR_OR_NULL(xen_obj))
> drivers/gpu/drm/xen/xen_drm_front_gem.c:120:    if (IS_ERR_OR_NULL(xen_obj->pages)) {
> drivers/gpu/drm/xen/xen_drm_front_gem.c:139:    if (IS_ERR_OR_NULL(xen_obj))
> drivers/gpu/drm/xen/xen_drm_front_gem.c:197:    if (IS_ERR_OR_NULL(xen_obj))
> drivers/gpu/drm/xen/xen_drm_front.c:403:        if (IS_ERR_OR_NULL(obj)) {
>
> They're all wrong, because if the pointer was really NULL then it's
> not handled correctly and would eventually lead to a runtime failure.

It seems that you were right and I can simply change all IS_ERR_OR_NULL 
to just IS_ERR

I am planning a series of patches later on, so I'll include this fix as well

>
> Normally Smatch is smart enough to know that the pointer isn't NULL so
> it doesn't generate a warning but yesterday I introduced a bug in Smatch
> by mistake.  It's fixed now.
>
> regards,
> dan carpenter
>
Thank you,

Oleksandr