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

[Gareth Stockwell <gareth.stockwell@xxxxxxx>]

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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