Modifying xenfb colour depth from Android domU

Modifying xenfb colour depth from Android domU

[Gareth Stockwell]

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:

int set_fb_pixel_format()
{
     int fd = open("/dev/graphics/fb0", O_RDWR, 0);

     struct fb_var_screeninfo info;
     if (ioctl(fd, FBIOGET_VSCREENINFO, &info) == -1) {
         return -errno;
     }

     info.red.offset = 11;
     info.red.length = 5;
     info.green.offset = 5;
     info.green.length = 6;
     info.blue.offset = 0;
     info.blue.length = 5;
     info.transp.offset = 0;
     info.transp.length = 0;
     info.bits_per_pixel = 16;

     info.activate = FB_ACTIVATE_NOW;

     if (ioctl(fd, FBIOPUT_VSCREENINFO, &info) == -1) {
         ALOGE("Selecting RGB565 failed");
         return -errno;
     }

     ...
}

This however fails in the xen-fbfront driver, due to bits_per_pixel not
matching the current framebuffer depth (which comes from #define
XENFB_DEPTH 32):

static int
xenfb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
{
      ...

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) {
         var->xres_virtual = var->xres;
         var->yres_virtual = var->yres;
         return 0;
     }
     return -EINVAL;
}

I found that I can fix this in one of two ways:

1. Change the initial depth to 16.  This of course means that guest
rendering in other depths is incorrectly displayed.

diff --git a/include/xen/interface/io/fbif.h
b/include/xen/interface/io/fbif.h
index 974a51e..42b1258 100644
--- a/include/xen/interface/io/fbif.h
+++ b/include/xen/interface/io/fbif.h
@@ -137,7 +137,7 @@ struct xenfb_page {
  #ifdef __KERNEL__
  #define XENFB_WIDTH 800
  #define XENFB_HEIGHT 600
-#define XENFB_DEPTH 32
+#define XENFB_DEPTH 16
  #endif

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?

Code being used:
linux: refs/tags/v3.10.16, plus "b1a3b1c xen/fb: allow xenfb
initialization for hvm guests"
qemu: refs/tags/v2.1.0-rc3
xen: refs/tags/RELEASE-4.4.0

Note that xenfb_check_var is unchanged in linux master.

Gareth


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Re: Modifying xenfb colour depth from Android domU

[David Vrabel]

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:

I'm not very familiar with the xenfb driver but it looks like it would
all just work provided you have a backend that supports feature-resize.

What backend are you using?

David

Re: Modifying xenfb colour depth from Android domU

[Gareth Stockwell]

On 28/08/14 11:15, 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:
> I'm not very familiar with the xenfb driver but it looks like it would
> all just work provided you have a backend that supports feature-resize.
>
> What backend are you using?
>
> David
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel
>

I've tried with both VNC and SDL backends.

If I read correctly the xenstore fragment below, it's reporting that the
VNC backend does support feature-resize, but FBIOPUT_VSCREENINFO still
fails as I described previously.

root@dom0:~# xenstore-ls /local/domain/0/backend/vfb
2 = ""
  0 = ""
   frontend = "/local/domain/2/device/vfb/0"
   frontend-id = "2"
   online = "1"
   state = "2"
   domain = "ree"
   vnc = "1"
   vnclisten = "0.0.0.0"
   vncdisplay = "2"
   vncunused = "1"
   sdl = "0"
   opengl = "0"
   feature-resize = "1"
   hotplug-status = "connected"

Gareth



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ARM Limited, Registered office 110 Fulbourn Road, Cambridge CB1 9NJ, Registered in England & Wales, Company No:  2557590
ARM Holdings plc, Registered office 110 Fulbourn Road, Cambridge CB1 9NJ, Registered in England & Wales, Company No:  2548782

Re: Modifying xenfb colour depth from Android domU

[David Vrabel]

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().

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

David

Re: Modifying xenfb colour depth from Android domU

[Stefano Stabellini]

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.