ARM: PL011: add DMA burst threshold support for ST variants

ST Micro variants has some specific dma burst threshold compensation,
which allows them to make better use of a DMA controller.  Add support
to set this up.

Based on a patch from Linus Walleij.

Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/serial/amba-pl011.c b/drivers/serial/amba-pl011.c
index ab025dc52fa4..e76d7d000128 100644
--- a/drivers/serial/amba-pl011.c
+++ b/drivers/serial/amba-pl011.c
@@ -74,6 +74,7 @@ struct vendor_data {
         unsigned int            lcrh_tx;
         unsigned int            lcrh_rx;
         bool                    oversampling;
+        bool                    dma_threshold;
 };
 
 static struct vendor_data vendor_arm = {
@@ -82,6 +83,7 @@ static struct vendor_data vendor_arm = {
         .lcrh_tx                = UART011_LCRH,
         .lcrh_rx                = UART011_LCRH,
         .oversampling           = false,
+        .dma_threshold          = false,
 };
 
 static struct vendor_data vendor_st = {
@@ -90,6 +92,7 @@ static struct vendor_data vendor_st = {
         .lcrh_tx                = ST_UART011_LCRH_TX,
         .lcrh_rx                = ST_UART011_LCRH_RX,
         .oversampling           = true,
+        .dma_threshold          = true,
 };
 
 /* Deals with DMA transactions */
@@ -527,6 +530,15 @@ static void pl011_dma_startup(struct uart_amba_port *uap)
         /* Turn on DMA error (RX/TX will be enabled on demand) */
         uap->dmacr |= UART011_DMAONERR;
         writew(uap->dmacr, uap->port.membase + UART011_DMACR);
+
+        /*
+         * ST Micro variants has some specific dma burst threshold
+         * compensation. Set this to 16 bytes, so burst will only
+         * be issued above/below 16 bytes.
+         */
+        if (uap->vendor->dma_threshold)
+                writew(ST_UART011_DMAWM_RX_16 | ST_UART011_DMAWM_TX_16,
+                               uap->port.membase + ST_UART011_DMAWM);
 }
 
 static void pl011_dma_shutdown(struct uart_amba_port *uap)