X-Git-Url: http://git.monstr.eu/?a=blobdiff_plain;f=drivers%2Fata%2Fahci_sunxi.c;h=cb69b737cb4996c9b1ca1643522b82b87c73f0fd;hb=0c01a4c4ca5b2b06bdc5f633b943eea5fe4f390e;hp=4100e904376b191e5770b7019b63460f74ff2821;hpb=c39f2d9db0fd81ea20bb5cce9b3f082ca63753e2;p=linux-2.6-microblaze.git

diff --git a/drivers/ata/ahci_sunxi.c b/drivers/ata/ahci_sunxi.c
index 4100e904376b..cb69b737cb49 100644
--- a/drivers/ata/ahci_sunxi.c
+++ b/drivers/ata/ahci_sunxi.c
@@ -149,8 +149,51 @@ static void ahci_sunxi_start_engine(struct ata_port *ap)
 	void __iomem *port_mmio = ahci_port_base(ap);
 	struct ahci_host_priv *hpriv = ap->host->private_data;
 
-	/* Setup DMA before DMA start */
-	sunxi_clrsetbits(hpriv->mmio + AHCI_P0DMACR, 0x0000ff00, 0x00004400);
+	/* Setup DMA before DMA start
+	 *
+	 * NOTE: A similar SoC with SATA/AHCI by Texas Instruments documents
+	 *   this Vendor Specific Port (P0DMACR, aka PxDMACR) in its
+	 *   User's Guide document (TMS320C674x/OMAP-L1x Processor
+	 *   Serial ATA (SATA) Controller, Literature Number: SPRUGJ8C,
+	 *   March 2011, Chapter 4.33 Port DMA Control Register (P0DMACR),
+	 *   p.68, https://www.ti.com/lit/ug/sprugj8c/sprugj8c.pdf)
+	 *   as equivalent to the following struct:
+	 *
+	 *   struct AHCI_P0DMACR_t
+	 *   {
+	 *     unsigned TXTS     : 4;
+	 *     unsigned RXTS     : 4;
+	 *     unsigned TXABL    : 4;
+	 *     unsigned RXABL    : 4;
+	 *     unsigned Reserved : 16;
+	 *   };
+	 *
+	 *   TXTS: Transmit Transaction Size (TX_TRANSACTION_SIZE).
+	 *     This field defines the DMA transaction size in DWORDs for
+	 *     transmit (system bus read, device write) operation. [...]
+	 *
+	 *   RXTS: Receive Transaction Size (RX_TRANSACTION_SIZE).
+	 *     This field defines the Port DMA transaction size in DWORDs
+	 *     for receive (system bus write, device read) operation. [...]
+	 *
+	 *   TXABL: Transmit Burst Limit.
+	 *     This field allows software to limit the VBUSP master read
+	 *     burst size. [...]
+	 *
+	 *   RXABL: Receive Burst Limit.
+	 *     Allows software to limit the VBUSP master write burst
+	 *     size. [...]
+	 *
+	 *   Reserved: Reserved.
+	 *
+	 *
+	 * NOTE: According to the above document, the following alternative
+	 *   to the code below could perhaps be a better option
+	 *   (or preparation) for possible further improvements later:
+	 *     sunxi_clrsetbits(hpriv->mmio + AHCI_P0DMACR, 0x0000ffff,
+	 *		0x00000033);
+	 */
+	sunxi_clrsetbits(hpriv->mmio + AHCI_P0DMACR, 0x0000ffff, 0x00004433);
 
 	/* Start DMA */
 	sunxi_setbits(port_mmio + PORT_CMD, PORT_CMD_START);