openwrt/target/linux/pistachio/patches-4.9/105-spi-img-spfi-RX-maximum-burst-size-for-DMA-is-8.patch

From 56466f505f58f44b69feb7eaed3b506842800456 Mon Sep 17 00:00:00 2001
From: Ionela Voinescu <ionela.voinescu@imgtec.com>
Date: Tue, 1 Mar 2016 17:49:45 +0000
Subject: spi: img-spfi: RX maximum burst size for DMA is 8

The depth of the FIFOs is 16 bytes. The DMA request line is tied
to the half full/empty (depending on the use of the TX or RX FIFO)
threshold. For the TX FIFO, if you set a burst size of 8 (equal to
half the depth) the first burst goes into FIFO without any issues,
but due the latency involved (the time the data leaves  the DMA
engine to the time it arrives at the FIFO), the DMA might trigger
another burst of 8. But given that there is no space for 2 additonal
bursts of 8, this would result in a failure. Therefore, we have to
keep the burst size for TX to 4 to accomodate for an extra burst.

For the read (RX) scenario, the DMA request line goes high when
there is at least 8 entries in the FIFO (half full), and we can
program the burst size to be 8 because the risk of accidental burst
does not exist. The DMA engine will not trigger another read until
the read data for all the burst it has sent out has been received.

While here, move the burst size setting outside of the if/else branches
as they have the same value for both 8 and 32 bit data widths.

Signed-off-by: Ionela Voinescu <ionela.voinescu@imgtec.com>
---
 drivers/spi/spi-img-spfi.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/drivers/spi/spi-img-spfi.c
+++ b/drivers/spi/spi-img-spfi.c
@@ -346,12 +346,11 @@ static int img_spfi_start_dma(struct spi
 		if (xfer->len % 4 == 0) {
 			rxconf.src_addr = spfi->phys + SPFI_RX_32BIT_VALID_DATA;
 			rxconf.src_addr_width = 4;
-			rxconf.src_maxburst = 4;
 		} else {
 			rxconf.src_addr = spfi->phys + SPFI_RX_8BIT_VALID_DATA;
 			rxconf.src_addr_width = 1;
-			rxconf.src_maxburst = 4;
 		}
+		rxconf.src_maxburst = 8;
 		dmaengine_slave_config(spfi->rx_ch, &rxconf);
 
 		rxdesc = dmaengine_prep_slave_sg(spfi->rx_ch, xfer->rx_sg.sgl,
@@ -370,12 +369,11 @@ static int img_spfi_start_dma(struct spi
 		if (xfer->len % 4 == 0) {
 			txconf.dst_addr = spfi->phys + SPFI_TX_32BIT_VALID_DATA;
 			txconf.dst_addr_width = 4;
-			txconf.dst_maxburst = 4;
 		} else {
 			txconf.dst_addr = spfi->phys + SPFI_TX_8BIT_VALID_DATA;
 			txconf.dst_addr_width = 1;
-			txconf.dst_maxburst = 4;
 		}
+		txconf.dst_maxburst = 4;
 		dmaengine_slave_config(spfi->tx_ch, &txconf);
 
 		txdesc = dmaengine_prep_slave_sg(spfi->tx_ch, xfer->tx_sg.sgl,