Re: [PATCH v3] dmaengine: qcom: gpi: Add GPI immediate DMA support for SPI protocol

[Jyothi Kumar Seerapu <quic_jseerapu@quicinc.com>]

On 12/4/2024 7:09 PM, Dmitry Baryshkov wrote:
> On Wed, 4 Dec 2024 at 15:25, Jyothi Kumar Seerapu
> <quic_jseerapu@quicinc.com> wrote:
>>
>>
>>
>> On 12/4/2024 6:15 PM, Dmitry Baryshkov wrote:
>>> On Wed, Dec 04, 2024 at 05:50:59PM +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@quicinc.com>
>>>> ---
>>>>
>>>> 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@quicinc.com/
>>>>       https://lore.kernel.org/all/20241128133351.24593-3-quic_jseerapu@quicinc.com/
>>>>
>>>> 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@quicinc.com/
>>>>
>>>>    drivers/dma/qcom/gpi.c | 32 +++++++++++++++++++++++++++-----
>>>>    1 file changed, 27 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/dma/qcom/gpi.c b/drivers/dma/qcom/gpi.c
>>>> index 52a7c8f2498f..35451d5a81f7 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;
>>>> +    int len;
>>>> +    u8 immediate_dma;
>>>>
>>>>       /* 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);
>>>> +    immediate_dma = (direction == DMA_MEM_TO_DEV) && len <= 2 * sizeof(tre->dword[0]);
>>>
>>> inline this condition, remove extra brackets and split the line after &&.
>> Hi Dmitry Baryshkov, thanks for the review.
>> Sure, i will make the changes mentioned below. Please let me know otherwise.
>>
>> immediate_dma = direction == DMA_MEM_TO_DEV &&
>>                   len <= 2 * sizeof(tre->dword[0]);
> 
> I was suggesting to _inline_ this condition rather than having a
> separate variable for it.

I can directly use the condition as follows:
if (direction == DMA_MEM_TO_DEV && len <= 2 * sizeof(tre->dword[0]))

However, this condition also needs to account for the 
"TRE_FLAGS_IMMEDIATE_DMA" update. Therefore, I introduced a separate 
variable.

tre->dword[3] |= u32_encode_bits(!!immediate_dma, TRE_FLAGS_IMMEDIATE_DMA);

Please let me know if it's acceptable to mention the entire condition in 
both places instead of using a separate variable.


> 
>>>> +
>>>> +    /* Support Immediate dma for write transfers for data length up to 8 bytes */
>>>> +    if (immediate_dma) {
>>>> +            /*
>>>> +             * 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(!!immediate_dma, 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
>>>>
>>>
> 
> 
>