[psplash][PATCH] psplash: Fix double buffering initialization

[psplash][PATCH] psplash: Fix double buffering initialization

[Horn, Michal]

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Some fb drivers do not implement double buffering completely or correctly.
For example mxsfb driver does not set yres_virtual to double on
initialization, nor it allows its doubling by FBIOPUT_VSCREENINFO ioctl
call. In such case, the double buffering gets enabled, but psplash fails
to display every second frame with error 'psplash_fb_flip: FBIOPAN_DISPLAY failed: Invalid argument'.

Panning the display by FBIOPAN_DISPLAY ioctl is always checking carefully
that the resolution, virtual buffer resolutin and offsets are in
bounds at fb_pan_display - https://elixir.bootlin.com/linux/v4.9.275/source/drivers/video/fbdev/core/fbmem.c#L891.

Swapping the front and back buffers is done by switching the
yoffset between 0 and yres values. For double buffering, the whole buffer
must have yres_virtual buffer size double of yres.

But in case of the mxsfb driver, the yres_virtual is always equal to yres,
so drawing with the offset set to yres would overrun the buffer. So the
panning is stopped and error 'Invalid argument' is returned.

Psplash tries to double the yres_virtual on initialization by FBIOPUT_VSCREENINFO ioctl call,
that at some point calls the mxsfb_check_var - https://elixir.bootlin.com/linux/v4.9.275/source/drivers/video/fbdev/mxsfb.c#L269
function, which for some reason always sets the yres_virtual back to yres,
effectively canceling the doubling. But no error is returned in this case,
so it gets silently ignored.

So in general, checking for the ioctl call success is not enough as some
drivers may pass this test, but fail double buffering anyway.
So now we double check the yres_virtual on fb_new and disable double
buffering in case of doubling yres_virtual fails.

Signed-off-by: Michal Horn <michalhorn@eaton.com>
---
psplash-fb.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/psplash-fb.c b/psplash-fb.c
index 2babb5f..1b73807 100644
--- a/psplash-fb.c
+++ b/psplash-fb.c
@@ -213,8 +213,18 @@ psplash_fb_new (int angle, int fbdev_id)
perror(" Error getting the fixed framebuffer info");
goto fail;
} else {
-          DBG("Virtual resolution set to double");
-          fb->double_buffering = 1;
+          if (ioctl(fb->fd, FBIOPAN_DISPLAY, &fb_var) == -1) {
+            fprintf(stderr, "warning: FBIOPAN_DISPLAY failed, "
+                    "double buffering disabled\n");
+          } else {
+            if (fb_var.yres_virtual == fb_var.yres * 2) {
+              DBG("Virtual resolution set to double");
+              fb->double_buffering = 1;
+            } else {
+              fprintf(stderr, "warning: Doubling virtual "
+                      "resolution failed, double buffering disabled\n");
+            }
+          }
}
}
}
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2.25.1

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[psplash][PATCH] psplash: Fix double buffering initialization

[Horn, Michal]

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Some fb drivers do not implement double buffering completely or correctly.
For example mxsfb driver does not set yres_virtual to double on
initialization, nor it allows its doubling by FBIOPUT_VSCREENINFO ioctl
call. In such case, the double buffering gets enabled, but psplash fails
to display every second frame with error
*psplash_fb_flip: FBIOPAN_DISPLAY failed: Invalid argument*.

# Technical details:

## Why every second frame gets thrown?

Panning the display by FBIOPAN_DISPLAY ioctl is always checking carefully
that the resolution, virtual buffer resolution and offsets in it are in
bounds at (fb_pan_display)[https://elixir.bootlin.com/linux/v4.9.275/source/drivers/video/fbdev/core/fbmem.c#L891].

Switching between the front and back buffers is done by switching the
yoffset between 0 and yres values. For double buffering, the complete buffer
has yres_virtual size and it must be double of yres.

But in case of the mxsfb driver, the yres_virtual is always equal to yres,
so drawing with the offset set to yres would overrun the buffer. So the
panning is stopped and error *Invalid argument* is returned.

## Why doubling the yres_virtual fails?

yres_virtual is supposed to be doubled by FBIOPUT_VSCREENINFO ioctl call,
that at some point calls the (mxsfb_check_var)[https://elixir.bootlin.com/linux/v4.9.275/source/drivers/video/fbdev/mxsfb.c#L269]
function, which for some reason always sets the yres_virtual back to yres,
effectively canceling the doubling. But no error is returned in this case,
so it gets silently ignored.

# Solution
These two problems can be solved by double checking the yres_virtual on
fb_new and disable double buffering in case of doubling yres_virtual fails.

Signed-off-by: Michal Horn <michalhorn@eaton.com>
---
psplash-fb.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/psplash-fb.c b/psplash-fb.c
index 2babb5f..1b73807 100644
--- a/psplash-fb.c
+++ b/psplash-fb.c
@@ -213,8 +213,18 @@ psplash_fb_new (int angle, int fbdev_id)
perror(" Error getting the fixed framebuffer info");
goto fail;
} else {
-          DBG("Virtual resolution set to double");
-          fb->double_buffering = 1;
+          if (ioctl(fb->fd, FBIOPAN_DISPLAY, &fb_var) == -1) {
+            fprintf(stderr, "warning: FBIOPAN_DISPLAY failed, "
+                    "double buffering disabled\n");
+          } else {
+            if (fb_var.yres_virtual == fb_var.yres * 2) {
+              DBG("Virtual resolution set to double");
+              fb->double_buffering = 1;
+            } else {
+              fprintf(stderr, "warning: Doubling virtual "
+                      "resolution failed, double buffering disabled\n");
+            }
+          }
}
}
}
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2.25.1

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