hw/display/bcm2835_fb: fix fb_use_offsets condition

It is common practice when implementing double-buffering on VideoCore
to do so by multiplying the height of the virtual buffer by the
number of virtual screens desired (i.e., two - in the case of
double-bufferring).

At present, this won't work in QEMU because the logic in
fb_use_offsets require that both the virtual width and height exceed
their physical counterparts.

This appears to be unintentional/a typo and indeed the comment
states; "Experimentally, the hardware seems to do this only if the
viewport size is larger than the physical screen".  The
viewport/virtual size would be larger than the physical size if
either virtual dimension were larger than their physical counterparts
and not necessarily both.

Signed-off-by: SamJakob <me@samjakob.com>
Message-id: 20240713160353.62410-1-me@samjakob.com
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/hw/display/bcm2835_fb.c b/hw/display/bcm2835_fb.c
index e40ed2d2e18f57c417d78ff291118503602657cb..650db3da82c2fe3b627a45666414310a0dc79495 100644 (file)
--- a/hw/display/bcm2835_fb.c
+++ b/hw/display/bcm2835_fb.c
@@ -145,7 +145,7 @@ static bool fb_use_offsets(BCM2835FBConfig *config)
      * viewport size is larger than the physical screen. (It doesn't
      * prevent the guest setting this silly viewport setting, though...)
      */
-    return config->xres_virtual > config->xres &&
+    return config->xres_virtual > config->xres ||
         config->yres_virtual > config->yres;
 }