[PATCH v3 2/2] usb: musb-new: sunxi: clarify the purpose of SRAM initialization

[Sam Edwards <cfsworks@gmail.com>]

This is largely a cosmetic change, with one functional distinction:
We are now only setting BIT(0), and no longer clearing BIT(1).

The A20 manual confirms the purpose and bitwidth of this field, and we
have also been doing it this way for a while in Linux-land: The prior
narrative about this initialization being about configuring a FIFO has
pretty much been debunked for years now.

This cleanup also adds a TODO comment about runtime discovery
of the SYSCON base, per discussion with Andre.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Cc: Andre Przywara <andre.przywara@arm.com>
---
 drivers/usb/musb-new/sunxi.c | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/drivers/usb/musb-new/sunxi.c b/drivers/usb/musb-new/sunxi.c
index 1111a67eaf..a8b1a8f870 100644
--- a/drivers/usb/musb-new/sunxi.c
+++ b/drivers/usb/musb-new/sunxi.c
@@ -171,15 +171,22 @@ static void USBC_ForceVbusValidToHigh(__iomem void *base)
 	musb_writel(base, USBC_REG_o_ISCR, reg_val);
 }
 
-static void USBC_ConfigFIFO_Base(void)
-{
-	u32 reg_value;
+/******************************************************************************
+ * Non-USBC register access needed for initialization
+ ******************************************************************************/
 
-	/* config usb fifo, 8kb mode */
-	reg_value = readl(SUNXI_SRAMC_BASE + 0x04);
-	reg_value &= ~(0x03 << 0);
-	reg_value |= BIT(0);
-	writel(reg_value, SUNXI_SRAMC_BASE + 0x04);
+/*
+ * A10(s), A13, GR8, A20:
+ * switch ownership of SRAM block 'D' to the USB-OTG controller
+ */
+static void sunxi_musb_claim_sram(uintptr_t syscon_base)
+{
+	/*
+	 * BIT(0) of SRAM_CTRL_REG1 (syscon+0x04) controls SRAM-D ownership:
+	 * '0' -> exclusive access by CPU
+	 * '1' -> exclusive access by USB0
+	 */
+	setbits_le32(syscon_base + 0x04, BIT(0));
 }
 
 /******************************************************************************
@@ -313,7 +320,13 @@ static int sunxi_musb_init(struct musb *musb)
 	musb->isr = sunxi_musb_interrupt;
 
 	if (glue->cfg->has_sram) {
-		USBC_ConfigFIFO_Base();
+		/*
+		 * This is an older USB-OTG controller that Allwinner did not
+		 * endow with a dedicated SRAM block; it instead uses SRAM
+		 * block 'D', ownership of which needs to be handed over by
+		 * the CPU
+		 */
+		sunxi_musb_claim_sram(SUNXI_SRAMC_BASE);
 	}
 
 	USBC_EnableDpDmPullUp(musb->mregs);
-- 
2.39.2