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

[Dmitry Baryshkov]

On Thu, Dec 05, 2024 at 10:36:11PM +0530, Jyothi Kumar Seerapu wrote:
> 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);

Don't repeat the condition, put it inside if.

> +	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
> 

-- 
With best wishes
Dmitry