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

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Message-ID: <fd7ea0d7-6e08-4000-b840-03e2da837e8d@quicinc.com>
Date: Thu, 5 Dec 2024 11:10:18 +0530
From: Jyothi Kumar Seerapu <quic_jseerapu@...cinc.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC: Vinod Koul <vkoul@...nel.org>, <linux-arm-msm@...r.kernel.org>,
        <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_msavaliy@...cinc.com>, <quic_vtanuku@...cinc.com>
Subject: Re: [PATCH v3] dmaengine: qcom: gpi: Add GPI immediate DMA support
 for SPI protocol



On 12/5/2024 3:53 AM, Dmitry Baryshkov wrote:
> On Wed, 4 Dec 2024 at 15:54, Jyothi Kumar Seerapu
> <quic_jseerapu@...cinc.com> wrote:
>>
>>
>>
>> On 12/4/2024 7:09 PM, Dmitry Baryshkov wrote:
>>> On Wed, 4 Dec 2024 at 15:25, Jyothi Kumar Seerapu
>>> <quic_jseerapu@...cinc.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@...cinc.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.
> 
> Move the flag setting under the if() too.
Sure thanks. I will make use of this condition directly instead using 
separate variable all over the applicable places.
> 
>>
>>
>>>
>>>>>> +
>>>>>> +    /* 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
>>>>>>
>>>>>
>>>
>>>
>>>
> 
> 
>