Re: [PATCH 5/9] drm/xen-front: Implement handling of shared display buffers

[Oleksandr Andrushchenko]

On 02/23/2018 04:36 PM, Boris Ostrovsky wrote:
> On 02/23/2018 02:53 AM, Oleksandr Andrushchenko wrote:
> > On 02/23/2018 02:25 AM, Boris Ostrovsky wrote:
> > > On 02/21/2018 03:03 AM, Oleksandr Andrushchenko wrote:
> > > >      static int __init xen_drv_init(void)
> > > >    {
> > > > +    /* At the moment we only support case with XEN_PAGE_SIZE ==
> > > > PAGE_SIZE */
> > > > +    BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
> > > Why BUILD_BUG_ON? This should simply not load if page sizes are
> > > different.
> > >
> > >    
> > This is a compile time check, so if kernel/Xen is configured
> > to use page size combination which is not supported by the
> > driver it will fail during compilation. This seems correct to me,
> > because you shouldn't even try to load the driver which
> > cannot handle different page sizes to not make any harm.
>
> This will prevent whole kernel from building. So, for example,
> randconfig builds will fail.
makes a lot of sense, thank you
will rework so I reject to load if the requirement is not met
> > > > +    ret = gnttab_map_refs(map_ops, NULL, buf->pages, buf->num_pages);
> > > > +    BUG_ON(ret);
> > > We should try not to BUG*(). There are a few in this patch (and possibly
> > > others) that I think can be avoided.
> > I will rework BUG_* for map/unmap code to handle errors,
> > but will still leave
> >      /* either pages or sgt, not both */
> >      BUG_ON(cfg->pages && cfg->sgt);
> > which is a real driver bug and must not happen
> Why not return an error?
>
> In fact, AFAICS you only call it in patch 9 where both of these can be
> tested, in which case something like -EINVAL would look reasonable.
ok, will remove BUG_ON as well
> -boris