[PATCH v4] dmaengine: qcom: gpi: Add GPI immediate DMA support for SPI protocol

[Jyothi Kumar Seerapu]

The DMA TRE(Transfer ring element) buffer contains the DMA
buffer address. Accessing data from this address can cause
significant delays in SPI transfers, which can be mitigated to
some extent by utilizing immediate DMA support.

QCOM GPI DMA hardware supports an immediate DMA feature for data
up to 8 bytes, storing the data directly in the DMA TRE buffer
instead of the DMA buffer address. This enhancement enables faster
SPI data transfers.

This optimization reduces the average transfer time from 25 us to
16 us for a single SPI transfer of 8 bytes length, with a clock
frequency of 50 MHz.

Signed-off-by: Jyothi Kumar Seerapu <quic_jseerapu@xxxxxxxxxxx>
---

v3 -> v4:
   - Instead using extra variable(immediate_dma) for Immediate dma
     condition check, made it to inlined.
   - Removed the extra brackets around Immediate dma condition check.

   Link to v3:
	https://lore.kernel.org/lkml/20241204122059.24239-1-quic_jseerapu@xxxxxxxxxxx/ 

v2 -> v3:
   - When to enable Immediate DMA support, control is moved to GPI driver
     from SPI driver.
   - Optimizations are done in GPI driver related to immediate dma changes.
   - Removed the immediate dma supported changes in qcom-gpi-dma.h file
     and handled in GPI driver.

   Link to v2:
	https://lore.kernel.org/all/20241128133351.24593-2-quic_jseerapu@xxxxxxxxxxx/
	https://lore.kernel.org/all/20241128133351.24593-3-quic_jseerapu@xxxxxxxxxxx/

v1 -> v2:
   - Separated the patches to dmaengine and spi subsystems
   - Removed the changes which are not required for this feature from
     qcom-gpi-dma.h file.
   - Removed the type conversions used in gpi_create_spi_tre.

   Link to v1:
	https://lore.kernel.org/lkml/20241121115201.2191-2-quic_jseerapu@xxxxxxxxxxx/

 drivers/dma/qcom/gpi.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/drivers/dma/qcom/gpi.c b/drivers/dma/qcom/gpi.c
index 52a7c8f2498f..9d4fc760bbe6 100644
--- a/drivers/dma/qcom/gpi.c
+++ b/drivers/dma/qcom/gpi.c
@@ -27,6 +27,7 @@
 #define TRE_FLAGS_IEOT		BIT(9)
 #define TRE_FLAGS_BEI		BIT(10)
 #define TRE_FLAGS_LINK		BIT(11)
+#define TRE_FLAGS_IMMEDIATE_DMA	BIT(16)
 #define TRE_FLAGS_TYPE		GENMASK(23, 16)
 
 /* SPI CONFIG0 WD0 */
@@ -64,6 +65,7 @@
 
 /* DMA TRE */
 #define TRE_DMA_LEN		GENMASK(23, 0)
+#define TRE_DMA_IMMEDIATE_LEN	GENMASK(3, 0)
 
 /* Register offsets from gpi-top */
 #define GPII_n_CH_k_CNTXT_0_OFFS(n, k)	(0x20000 + (0x4000 * (n)) + (0x80 * (k)))
@@ -1711,6 +1713,7 @@ static int gpi_create_spi_tre(struct gchan *chan, struct gpi_desc *desc,
 	dma_addr_t address;
 	struct gpi_tre *tre;
 	unsigned int i;
+	int len;
 
 	/* first create config tre if applicable */
 	if (direction == DMA_MEM_TO_DEV && spi->set_config) {
@@ -1763,14 +1766,32 @@ static int gpi_create_spi_tre(struct gchan *chan, struct gpi_desc *desc,
 	tre_idx++;
 
 	address = sg_dma_address(sgl);
-	tre->dword[0] = lower_32_bits(address);
-	tre->dword[1] = upper_32_bits(address);
+	len = sg_dma_len(sgl);
 
-	tre->dword[2] = u32_encode_bits(sg_dma_len(sgl), TRE_DMA_LEN);
+	/* Support Immediate dma for write transfers for data length up to 8 bytes */
+	if (direction == DMA_MEM_TO_DEV && len <= 2 * sizeof(tre->dword[0])) {
+		/*
+		 * For Immediate dma, data length may not always be length of 8 bytes,
+		 * it can be length less than 8, hence initialize both dword's with 0
+		 */
+		tre->dword[0] = 0;
+		tre->dword[1] = 0;
+		memcpy(&tre->dword[0], sg_virt(sgl), len);
+
+		tre->dword[2] = u32_encode_bits(len, TRE_DMA_IMMEDIATE_LEN);
+	} else {
+		tre->dword[0] = lower_32_bits(address);
+		tre->dword[1] = upper_32_bits(address);
+
+		tre->dword[2] = u32_encode_bits(len, TRE_DMA_LEN);
+	}
 
 	tre->dword[3] = u32_encode_bits(TRE_TYPE_DMA, TRE_FLAGS_TYPE);
-	if (direction == DMA_MEM_TO_DEV)
-		tre->dword[3] |= u32_encode_bits(1, TRE_FLAGS_IEOT);
+	tre->dword[3] |= u32_encode_bits(direction == DMA_MEM_TO_DEV &&
+					 len <= 2 * sizeof(tre->dword[0]),
+					 TRE_FLAGS_IMMEDIATE_DMA);
+	tre->dword[3] |= u32_encode_bits(direction == DMA_MEM_TO_DEV,
+					 TRE_FLAGS_IEOT);
 
 	for (i = 0; i < tre_idx; i++)
 		dev_dbg(dev, "TRE:%d %x:%x:%x:%x\n", i, desc->tre[i].dword[0],
-- 
2.17.1