cirrusfb: do not change MCLK for Alpine chips

A memory clock value (MCLK) is changed to a minimum required by a current
mode bandwidth.  This usually lowers the MCLK to its minimum (50 MHz) thus
decreasing the card performance.  Just leave the MCLK value set by card
BIOS.

The CL-GD5446 Technical Reference Manual point 9.9.1.3 states that if a
pixclock value is close (~1%) to the MCLK or MCLK/2 this may result in a
jitter on the screen.  A countermeasure is to use the MCLK as pixclock
source instead of a VCLK.  The patch implements this as well.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 53 insertions, 66 deletions

diff --git a/drivers/video/cirrusfb.c b/drivers/video/cirrusfb.c
index 9e0a1c58fde4..048b139f0e50 100644
--- a/drivers/video/cirrusfb.c
+++ b/drivers/video/cirrusfb.c
@@ -327,9 +327,7 @@ static const struct {
 #endif /* CONFIG_ZORRO */
 
 struct cirrusfb_regs {
-        long multiplexing;
-        long mclk;
-        long divMCLK;
+        int multiplexing;
 };
 
 #ifdef CIRRUSFB_DEBUG
@@ -461,45 +459,28 @@ static int cirrusfb_release(struct fb_info *info, int user)
 /****************************************************************************/
 /**** BEGIN Hardware specific Routines **************************************/
 
-/* Get a good MCLK value */
-static long cirrusfb_get_mclk(long freq, int bpp, long *div)
+/* Check if the MCLK is not a better clock source */
+static int cirrusfb_check_mclk(struct cirrusfb_info *cinfo, long freq)
 {
-        long mclk;
+        long mclk = vga_rseq(cinfo->regbase, CL_SEQR1F) & 0x3f;
 
-        assert(div != NULL);
-
-        /* Calculate MCLK, in case VCLK is high enough to require > 50MHz.
-         * Assume a 64-bit data path for now.  The formula is:
-         * ((B * PCLK * 2)/W) * 1.2
-         * B = bytes per pixel, PCLK = pixclock, W = data width in bytes */
-        mclk = ((bpp / 8) * freq * 2) / 4;
-        mclk = (mclk * 12) / 10;
-        if (mclk < 50000)
-                mclk = 50000;
-        DPRINTK("Use MCLK of %ld kHz\n", mclk);
-
-        /* Calculate value for SR1F.  Multiply by 2 so we can round up. */
-        mclk = ((mclk * 16) / 14318);
-        mclk = (mclk + 1) / 2;
-        DPRINTK("Set SR1F[5:0] to 0x%lx\n", mclk);
+        /* Read MCLK value */
+        mclk = (14318 * mclk) >> 3;
+        DPRINTK("Read MCLK of %ld kHz\n", mclk);
 
         /* Determine if we should use MCLK instead of VCLK, and if so, what we
-           * should divide it by to get VCLK */
-        switch (freq) {
-        case 24751 ... 25249:
-                *div = 2;
-                DPRINTK("Using VCLK = MCLK/2\n");
-                break;
-        case 49501 ... 50499:
-                *div = 1;
+         * should divide it by to get VCLK
+         */
+
+        if (abs(freq - mclk) < 250) {
                 DPRINTK("Using VCLK = MCLK\n");
-                break;
-        default:
-                *div = 0;
-                break;
+                return 1;
+        } else if (abs(freq - (mclk / 2)) < 250) {
+                DPRINTK("Using VCLK = MCLK/2\n");
+                return 2;
         }
 
-        return mclk;
+        return 0;
 }
 
 static int cirrusfb_check_var(struct fb_var_screeninfo *var,
@@ -705,30 +686,27 @@ static int cirrusfb_decode_var(const struct fb_var_screeninfo *var,
                 break;
         }
 #endif
-        regs->mclk = cirrusfb_get_mclk(freq, var->bits_per_pixel,
-                                        &regs->divMCLK);
-
         return 0;
 }
 
-static void cirrusfb_set_mclk(const struct cirrusfb_info *cinfo, int val,
-                                int div)
+static void cirrusfb_set_mclk_as_source(const struct cirrusfb_info *cinfo,
+                                        int div)
 {
+        unsigned char old1f, old1e;
         assert(cinfo != NULL);
+        old1f = vga_rseq(cinfo->regbase, CL_SEQR1F) & ~0x40;
 
-        if (div == 2) {
-                /* VCLK = MCLK/2 */
-                unsigned char old = vga_rseq(cinfo->regbase, CL_SEQR1E);
-                vga_wseq(cinfo->regbase, CL_SEQR1E, old | 0x1);
-                vga_wseq(cinfo->regbase, CL_SEQR1F, 0x40 | (val & 0x3f));
-        } else if (div == 1) {
-                /* VCLK = MCLK */
-                unsigned char old = vga_rseq(cinfo->regbase, CL_SEQR1E);
-                vga_wseq(cinfo->regbase, CL_SEQR1E, old & ~0x1);
-                vga_wseq(cinfo->regbase, CL_SEQR1F, 0x40 | (val & 0x3f));
-        } else {
-                vga_wseq(cinfo->regbase, CL_SEQR1F, val & 0x3f);
+        if (div) {
+                DPRINTK("Set %s as pixclock source.\n",
+                                        (div == 2) ? "MCLK/2" : "MCLK");
+                old1f |= 0x40;
+                old1e = vga_rseq(cinfo->regbase, CL_SEQR1E) & ~0x1;
+                if (div == 2)
+                        old1e |= 1;
+
+                vga_wseq(cinfo->regbase, CL_SEQR1E, old1e);
         }
+        vga_wseq(cinfo->regbase, CL_SEQR1F, old1f);
 }
 
 /*************************************************************************
@@ -904,19 +882,31 @@ static int cirrusfb_set_par_foo(struct fb_info *info)
         /* formula: VClk = (OSC * N) / (D * (1+P)) */
         /* Example: VClk = (14.31818 * 91) / (23 * (1+1)) = 28.325 MHz */
 
-        vga_wseq(regbase, CL_SEQRB, nom);
-        tmp = den << 1;
-        if (div != 0)
-                tmp |= 1;
+        if (cinfo->btype == BT_ALPINE) {
+                /* if freq is close to mclk or mclk/2 select mclk
+                 * as clock source
+                 */
+                int divMCLK = cirrusfb_check_mclk(cinfo, freq);
+                if (divMCLK)  {
+                        nom = 0;
+                        cirrusfb_set_mclk_as_source(cinfo, divMCLK);
+                }
+        }
+        if (nom) {
+                vga_wseq(regbase, CL_SEQRB, nom);
+                tmp = den << 1;
+                if (div != 0)
+                        tmp |= 1;
 
-        /* 6 bit denom; ONLY 5434!!! (bugged me 10 days) */
-        if ((cinfo->btype == BT_SD64) ||
-            (cinfo->btype == BT_ALPINE) ||
-            (cinfo->btype == BT_GD5480))
-                tmp |= 0x80;
+                /* 6 bit denom; ONLY 5434!!! (bugged me 10 days) */
+                if ((cinfo->btype == BT_SD64) ||
+                    (cinfo->btype == BT_ALPINE) ||
+                    (cinfo->btype == BT_GD5480))
+                        tmp |= 0x80;
 
-        DPRINTK("CL_SEQR1B: %ld\n", (long) tmp);
-        vga_wseq(regbase, CL_SEQR1B, tmp);
+                DPRINTK("CL_SEQR1B: %ld\n", (long) tmp);
+                vga_wseq(regbase, CL_SEQR1B, tmp);
+        }
 
         if (yres >= 1024)
                 /* 1280x1024 */
@@ -1106,7 +1096,6 @@ static int cirrusfb_set_par_foo(struct fb_info *info)
 
                 case BT_ALPINE:
                         DPRINTK(" (for GD543x)\n");
-                        cirrusfb_set_mclk(cinfo, regs.mclk, regs.divMCLK);
                         /* We already set SRF and SR1F */
                         break;
 
@@ -1179,7 +1168,6 @@ static int cirrusfb_set_par_foo(struct fb_info *info)
                 case BT_ALPINE:
                         DPRINTK(" (for GD543x)\n");
                         vga_wseq(regbase, CL_SEQR7, 0xa7);
-                        cirrusfb_set_mclk(cinfo, regs.mclk, regs.divMCLK);
                         break;
 
                 case BT_GD5480:
@@ -1257,7 +1245,6 @@ static int cirrusfb_set_par_foo(struct fb_info *info)
                 case BT_ALPINE: