From 5fff854126b0367168bdcbb24f6c49f053c045ee Mon Sep 17 00:00:00 2001
From: Thom Johansen <thomj@rockbox.org>
Date: Wed, 1 Feb 2006 00:04:55 +0000
Subject: EQ filtering code for ARM targets.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@8515 a1c6a512-1295-4272-9138-f99709370657
---
 apps/eq_arm.S | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 apps/eq_arm.S

diff --git a/apps/eq_arm.S b/apps/eq_arm.S
new file mode 100644
index 0000000000..85617dc2fb
--- /dev/null
+++ b/apps/eq_arm.S
@@ -0,0 +1,67 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2006 Thom Johansen
+ *
+ * All files in this archive are subject to the GNU General Public License.
+ * See the file COPYING in the source tree root for full license agreement.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+
+    .text
+    .global eq_filter
+eq_filter:
+    ldr r12, [sp]             @ get shift parameter
+    stmdb sp!, { r0-r11, lr } @ save all params and clobbered regs 
+    ldmia r1!, { r4-r8 }      @ load coefs
+    mov r10, r1               @ loop prelude expects filter struct addr in r10
+
+.filterloop:
+    ldr r9, [sp]            @ get pointer to this channels data
+    add r0, r9, #4
+    str r0, [sp]            @ save back pointer to next channels data
+    ldr r9, [r9]            @ r9 = x[]
+    ldr r14, [sp, #8]       @ r14 = numsamples
+    ldmia r10, { r0-r3 }    @ load history, r10 should be filter struct addr
+    str r10, [sp, #4]       @ save it for loop end
+.loop:
+    /* r0-r3 = history, r4-r8 = coefs, r9 = x[], r10..r11 = accumulator,
+       r12 = shift amount, r14 = number of samples.
+       See eq_cf.S for explanation of what this loop does. Primary difference
+       is the reordering of the equation we do here, which is done for register
+       reuse reasons, we're pretty short on regs.
+     */
+    smull r10, r11, r6, r1  @ acc = b2*x[i - 2]
+    mov r1, r0              @ fix input history
+    smlal r10, r11, r5, r0  @ acc += b1*x[i - 1]
+    ldr r0, [r9]            @ load input and fix history in same operation
+    smlal r10, r11, r4, r0  @ acc += b0*x[i]
+    smlal r10, r11, r7, r2  @ acc += a1*y[i - 1]
+    smlal r10, r11, r8, r3  @ acc += a2*y[i - 2]
+    mov r3, r2              @ fix output history
+    mov r2, r11, lsl r12    @ get result
+    @ TODO: arm makes it easy to mix in lower bits from r10 for extended
+    @ precision here, but we don't have enough regs to save the shift factor
+    @ we would need (32 - r12).
+    str r2, [r9], #4        @ save result
+    subs r14, r14, #1       @ are we done with this channel?
+    bne .loop
+
+    ldr r10, [sp, #4]       @ load filter struct pointer
+    stmia r10!, { r0-r3 }   @ save back history
+    ldr r11, [sp, #12]      @ load number of channels
+    subs r11, r11, #1       @ all channels processed?
+    strne r11, [sp, #12]
+    bne .filterloop
+
+    add sp, sp, #16         @ compensate for temp storage
+    ldmia sp!, { r4-r11, pc }
-- 
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