[PATCH] mbxfb: Fix a chip bug? resulting in wrong pixclock

This is a workaround for what I think is a bug in the 2700G chip.

The PLL output frequency is adustable using 3 values (M, N and P.  See code
for formula).  The N value range is documented to be 1 to 7 but when it is set
to 1, the output frequency is lower than it should be (divided by 2), giving
unexpected results such as no sync on a CRT display.

This patch prevents N=1 when searching for the best value for the requested
pixclock.

Signed-off-by: Raphael Assenat <raph@8d.com>
Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/drivers/video/mbx/mbxfb.c b/drivers/video/mbx/mbxfb.c
index 6849ab75d403..cfc6bf3615b5 100644
--- a/drivers/video/mbx/mbxfb.c
+++ b/drivers/video/mbx/mbxfb.c
@@ -118,8 +118,19 @@ static unsigned int mbxfb_get_pixclock(unsigned int pixclock_ps,
         /* convert pixclock to KHz */
         pixclock = PICOS2KHZ(pixclock_ps);
 
+        /* PLL output freq = (ref_clk * M) / (N * 2^P)
+         *
+         * M: 1 to 63
+         * N: 1 to 7
+         * P: 0 to 7
+         */
+
+        /* RAPH: When N==1, the resulting pixel clock appears to
+         * get divided by 2. Preventing N=1 by starting the following
+         * loop at 2 prevents this. Is this a bug with my chip
+         * revision or something I dont understand? */
         for (m = 1; m < 64; m++) {
-                for (n = 1; n < 8; n++) {
+                for (n = 2; n < 8; n++) {
                         for (p = 0; p < 8; p++) {
                                 clk = (ref_clk * m) / (n * (1 << p));
                                 err = (clk > pixclock) ? (clk - pixclock) :