Re: [Xen-devel] Modifying xenfb colour depth from Android domU

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 28 Aug 2014, David Vrabel wrote:
> On 28/08/14 10:21, Gareth Stockwell wrote:
> > I'm facing some problems getting xenfb to work with an Android domU, on
> > ARM.
> > 
> > The issue is that the framebuffer is created with a depth of 32bpp,
> > while the Android guest is rendering RGB565.
> > 
> > I tried to fix this by using the fbdev API to request a change of pixel
> > format, during the Android startup:
> 
> Looked at this more closely.
> 
> > 2. Allow the guest to modify the framebuffer depth.
> > 
> > diff --git a/drivers/video/xen-fbfront.c b/drivers/video/xen-fbfront.c
> > index 02e1c01..b65a485 100644
> > --- a/drivers/video/xen-fbfront.c
> > +++ b/drivers/video/xen-fbfront.c
> > @@ -295,12 +295,11 @@ xenfb_check_var(struct fb_var_screeninfo *var,
> > struct fb_info *info)
> >         if (var->xres > video[KPARAM_WIDTH] || var->yres >
> > video[KPARAM_HEIGHT])
> >                 return -EINVAL;
> > 
> > -       required_mem_len = var->xres * var->yres *
> > xenfb_info->page->depth / 8;
> > -       if (var->bits_per_pixel == xenfb_info->page->depth &&
> > -           var->xres <= info->fix.line_length / (XENFB_DEPTH / 8) &&
> > -           required_mem_len <= info->fix.smem_len) {
> > +       required_mem_len = var->xres * var->yres * var->bits_per_pixel / 8;
> > +       if (required_mem_len <= info->fix.smem_len) {
> >                 var->xres_virtual = var->xres;
> >                 var->yres_virtual = var->yres;
> > +               info->fix.line_length = var->xres * var->bits_per_pixel
> > / 8;
> >                 return 0;
> >         }
> >         return -EINVAL;
> > 
> > This works for my case, but involves modifying the
> > fb_info.fix.line_length value - is this allowed?
> 
> You must not in change it in xenfb_check_var() since is only for
> validating the mode. Instead you should change it in xenfb_set_par().

As David wrote, yes, you are allowed to change info->fix.line_length,
but you need to do it from xenfb_set_par.
Of course you also need to fix the check in xenfb_check_var.


> When posting the patch, please Cc the framebuffer list and maintainers
> in addition to xen-devel.

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