drivers: ata: ahci_sunxi: Increased SATA/AHCI DMA TX/RX FIFOs

Increasing the SATA/AHCI DMA TX/RX FIFOs (P0DMACR.TXTS and .RXTS, ie.
TX_TRANSACTION_SIZE and RX_TRANSACTION_SIZE) from default 0x0 each
to 0x3 each, gives a write performance boost of 120 MiB/s to 132 MiB/s
from lame 36 MiB/s to 45 MiB/s previously.
Read performance is above 200 MiB/s.
[tested on SSD using dd bs=4K/8K/12K/16K/20K/24K/32K: peak-perf at 12K]

Tested on the SBCs Banana Pi R1 (aka Lamobo R1) and Banana Pi M1 which
are based on the Allwinner A20 32bit-SoC (ARMv7-a / arm-linux-gnueabihf).
These devices are RaspberryPi-like small devices.

This problem of slow SATA write-speed with these small devices lasts
for about 7 years now (beginning with the A10 SoC). Many commentators
throughout the years wrongly assumed the slow write speed was a
hardware limitation. This patch finally solves the problem, which
in fact was just a hard-to-find software problem due to lack of
SATA/AHCI documentation by the SoC-maker Allwinner Technology.

Lists of the affected sunxi and other boards and SoCs with SATA using
the ahci_sunxi driver:
  $ grep -i -e "^&ahci" arch/arm/boot/dts/sun*dts
  and http://linux-sunxi.org/SATA#Devices_with_SATA_ports
  See also http://linux-sunxi.org/Category:Devices_with_SATA_port

Tested-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Uenal Mutlu <um@mutluit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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);