On Thu, Nov 28, 2024 at 07:03:50PM +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>
> ---
> 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.
>
> drivers/dma/qcom/gpi.c | 32 +++++++++++++++++++++++++++-----
> include/linux/dma/qcom-gpi-dma.h | 6 ++++++
> 2 files changed, 33 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/dma/qcom/gpi.c b/drivers/dma/qcom/gpi.c
> index 52a7c8f2498f..4c5df696ddd8 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 +1713,8 @@ static int gpi_create_spi_tre(struct gchan *chan, struct gpi_desc *desc,
> dma_addr_t address;
> struct gpi_tre *tre;
> unsigned int i;
> + u8 *buf;
> + int len = 0;
First use of "len" is an assignment, so you shouldn't zero-initialize it
here.
>
> /* first create config tre if applicable */
> if (direction == DMA_MEM_TO_DEV && spi->set_config) {
> @@ -1763,14 +1767,32 @@ 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 */
And what happens if the developer writing the SPI driver forgets to read
this comment and sets QCOM_GPI_IMMEDIATE_DMA for a 9 byte transfer?
> + if ((spi->flags & QCOM_GPI_IMMEDIATE_DMA) && direction == DMA_MEM_TO_DEV) {
Why is this flag introduced?
If I understand the next patch, all DMA_MEM_TO_DEV transfers of <=
QCOM_GPI_IMMEDIATE_DMA_LEN can use the immediate mode, so why not move
the condition here?
Also ordering[1].
if (direction == DMA_MEM_TO_DEV && len <= 2 * sizeof(tre->dword[0]))
[1] Compare "all transfers of length 8 or less, which are mem to device"
vs "all transfers which are mem to device, with a length of 8 or less".
The bigger "selection criteria" is the direction, then that's fine tuned
by the length query.
> + buf = sg_virt(sgl);
It's a question of style, but I think you could just put the sg_virt()
directly in the memcpy() call and avoid the extra variable.
>
> - tre->dword[3] = u32_encode_bits(TRE_TYPE_DMA, TRE_FLAGS_TYPE);
> - if (direction == DMA_MEM_TO_DEV)
> + /* memcpy may not always be length of 8, hence pre-fill both dword's with 0 */
The implementation of memcpy() is always more than 8 bytes, it's buf
that might be less than 8 bytes ;)
Also you're not "pre-filling", you're "zero-initializing", or just
"initialize".
That said, does it matter? Will the QUP read beyond the
TRE_DMA_IMMEDIATE_LEN bytes? If so, please put _that_ in the comment
("QUP reads beyond the provided len, so additional content needs to be
cleared", or similar)
> + tre->dword[0] = 0;
> + tre->dword[1] = 0;
> + memcpy(&tre->dword[0], buf, len);
> +
> + tre->dword[2] = u32_encode_bits(len, TRE_DMA_IMMEDIATE_LEN);
Does the format of tre->dword[2] really change when
TRE_FLAGS_IMMEDIATE_DMA is set, or is TRE_DMA_IMMEDIATE_LEN just a
mask to highlight that len can't be more than 4 bits?
It seems like you could drop TRE_DMA_IMMEDIATE_LEN and just use
TRE_DMA_LEN here? (But it should match what the hardware programming
guide states)
Perhaps you could reduce the scope of this if/else then as well, as the
assignment of of dword[2] and dword[3] is mostly the same with and
without immediate mode (just the one bit to enable it)
> +
> + tre->dword[3] = u32_encode_bits(TRE_TYPE_DMA, TRE_FLAGS_TYPE);
> tre->dword[3] |= u32_encode_bits(1, TRE_FLAGS_IEOT);
> + tre->dword[3] |= u32_encode_bits(1, TRE_FLAGS_IMMEDIATE_DMA);
> + } 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);
> + }
>
> for (i = 0; i < tre_idx; i++)
> dev_dbg(dev, "TRE:%d %x:%x:%x:%x\n", i, desc->tre[i].dword[0],
> diff --git a/include/linux/dma/qcom-gpi-dma.h b/include/linux/dma/qcom-gpi-dma.h
> index 6680dd1a43c6..84598848d53a 100644
> --- a/include/linux/dma/qcom-gpi-dma.h
> +++ b/include/linux/dma/qcom-gpi-dma.h
> @@ -15,6 +15,10 @@ enum spi_transfer_cmd {
> SPI_DUPLEX,
> };
>
> +#define QCOM_GPI_IMMEDIATE_DMA BIT(1)
> +
> +#define QCOM_GPI_IMMEDIATE_DMA_LEN 8
> +
> /**
> * struct gpi_spi_config - spi config for peripheral
> *
> @@ -30,6 +34,7 @@ enum spi_transfer_cmd {
> * @cs: chip select toggle
> * @set_config: set peripheral config
> * @rx_len: receive length for buffer
> + * @flags: true for immediate dma support
Per above I think you can remove this flag, but "true for immediate DMA
support" doesn't match what you have written in the code. (Also in
general u8 shouldn't be "true")
Regards,
Bjorn
> */
> struct gpi_spi_config {
> u8 set_config;
> @@ -44,6 +49,7 @@ struct gpi_spi_config {
> u32 clk_src;
> enum spi_transfer_cmd cmd;
> u32 rx_len;
> + u8 flags;
> };
>
> enum i2c_op {
> --
> 2.17.1
>
>
