sh: dma: Fix DMA channel offset calculation

Various SoCs of the SH3, SH4 and SH4A family, which use this driver,
feature a differing number of DMA channels, which can be distributed
between up to two DMAC modules. The existing implementation fails to
correctly accommodate for all those variations, resulting in wrong
channel offset calculations and leading to kernel panics.

Rewrite dma_base_addr() in order to properly calculate channel offsets
in a DMAC module. Fix dmaor_read_reg() and dmaor_write_reg(), so that
the correct DMAC module base is selected for the DMAOR register.

Fixes: 7f47c718 ("sh: dma: More legacy cpu dma chainsawing.")
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230527164452.64797-2-contact@artur-rojek.euSigned-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

arch/sh/drivers/dma/dma-sh.c

@@ -18,6 +18,18 @@
 #include <cpu/dma-register.h>
 #include <cpu/dma.h>
 
+/*
+ * Some of the SoCs feature two DMAC modules. In such a case, the channels are
+ * distributed equally among them.
+ */
+#ifdef	SH_DMAC_BASE1
+#define	SH_DMAC_NR_MD_CH	(CONFIG_NR_ONCHIP_DMA_CHANNELS / 2)
+#else
+#define	SH_DMAC_NR_MD_CH	CONFIG_NR_ONCHIP_DMA_CHANNELS
+#endif
+
+#define	SH_DMAC_CH_SZ		0x10
+
 /*
  * Define the default configuration for dual address memory-memory transfer.
  * The 0x400 value represents auto-request, external->external.
@@ -29,7 +41,7 @@ static unsigned long dma_find_base(unsigned int chan)
 	unsigned long base = SH_DMAC_BASE0;
 
 #ifdef SH_DMAC_BASE1
-	if (chan >= 6)
+	if (chan >= SH_DMAC_NR_MD_CH)
 		base = SH_DMAC_BASE1;
 #endif
 
@@ -40,13 +52,13 @@ static unsigned long dma_base_addr(unsigned int chan)
 {
 	unsigned long base = dma_find_base(chan);
 
-	/* Normalize offset calculation */
-	if (chan >= 9)
-		chan -= 6;
-	if (chan >= 4)
-		base += 0x10;
+	chan = (chan % SH_DMAC_NR_MD_CH) * SH_DMAC_CH_SZ;
+
+	/* DMAOR is placed inside the channel register space. Step over it. */
+	if (chan >= DMAOR)
+		base += SH_DMAC_CH_SZ;
 
-	return base + (chan * 0x10);
+	return base + chan;
 }
 
 #ifdef CONFIG_SH_DMA_IRQ_MULTI
@@ -250,12 +262,11 @@ static int sh_dmac_get_dma_residue(struct dma_channel *chan)
 #define NR_DMAOR	1
 #endif
 
-/*
- * DMAOR bases are broken out amongst channel groups. DMAOR0 manages
- * channels 0 - 5, DMAOR1 6 - 11 (optional).
- */
-#define dmaor_read_reg(n)		__raw_readw(dma_find_base((n)*6))
-#define dmaor_write_reg(n, data)	__raw_writew(data, dma_find_base(n)*6)
+#define dmaor_read_reg(n)		__raw_readw(dma_find_base((n) * \
+						    SH_DMAC_NR_MD_CH) + DMAOR)
+#define dmaor_write_reg(n, data)	__raw_writew(data, \
+						     dma_find_base((n) * \
+						     SH_DMAC_NR_MD_CH) + DMAOR)
 
 static inline int dmaor_reset(int no)
 {