Re: [PATCH v4 5/9] spi: spi-mem: Mark dummy transfers by setting dummy_data bit

[Sowjanya Komatineni <skomatineni@nvidia.com>]

On 12/18/20 1:57 AM, Boris Brezillon wrote:
> On Fri, 18 Dec 2020 14:51:08 +0530
> Pratyush Yadav <p.yadav@ti.com> wrote:
>
>> Hi Sowjanya,
>>
>> On 17/12/20 12:28PM, Sowjanya Komatineni wrote:
>>> This patch marks dummy transfer by setting dummy_data bit to 1.
>>>
>>> Controllers supporting dummy transfer by hardware use this bit field
>>> to skip software transfer of dummy bytes and use hardware dummy bytes
>>> transfer.
>> What is the benefit you get from this change? You add complexity in
>> spi-mem and the controller driver, so that must come with some benefits.
>> Here I don't see any. The transfer will certainly take the same amount
>> of time because the number or period of the dummy cycles has not
>> changed. So why is this needed?
> Well, you don't have to queue TX bytes if you use HW-based dummy
> cycles, but I agree, I'd expect the overhead to be negligible,
> especially since we're talking about emitting a few bytes, not hundreds.
> This being said, the complexity added to the core is reasonable IMHO,
> so if it really helps reducing the CPU overhead (we might need some
> numbers to prove that), I guess it's okay.

Hardware dummy cycles feature of Tegra QSPI is to save SW transfer cycle 
of dummy bytes by filling FIFO.

I don't have numbers as we always use hardware dummy cycles with Tegra QSPI.

>>   
>>> Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
>>> ---
>>>   drivers/spi/spi-mem.c   | 1 +
>>>   include/linux/spi/spi.h | 2 ++
>>>   2 files changed, 3 insertions(+)
>>>
>>> diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
>>> index f3a3f19..c64371c 100644
>>> --- a/drivers/spi/spi-mem.c
>>> +++ b/drivers/spi/spi-mem.c
>>> @@ -354,6 +354,7 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
>>>   		xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
>>>   		xfers[xferpos].len = op->dummy.nbytes;
>>>   		xfers[xferpos].tx_nbits = op->dummy.buswidth;
>>> +		xfers[xferpos].dummy_data = 1;
>>>   		spi_message_add_tail(&xfers[xferpos], &msg);
>>>   		xferpos++;
>>>   		totalxferlen += op->dummy.nbytes;
>>> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
>>> index aa09fdc..708f2f5 100644
>>> --- a/include/linux/spi/spi.h
>>> +++ b/include/linux/spi/spi.h
>>> @@ -827,6 +827,7 @@ extern void spi_res_release(struct spi_controller *ctlr,
>>>    *      transfer. If 0 the default (from @spi_device) is used.
>>>    * @bits_per_word: select a bits_per_word other than the device default
>>>    *      for this transfer. If 0 the default (from @spi_device) is used.
>>> + * @dummy_data: indicates transfer is dummy bytes transfer.
>>>    * @cs_change: affects chipselect after this transfer completes
>>>    * @cs_change_delay: delay between cs deassert and assert when
>>>    *      @cs_change is set and @spi_transfer is not the last in @spi_message
>>> @@ -939,6 +940,7 @@ struct spi_transfer {
>>>   	struct sg_table tx_sg;
>>>   	struct sg_table rx_sg;
>>>   
>>> +	unsigned	dummy_data:1;
>>>   	unsigned	cs_change:1;
>>>   	unsigned	tx_nbits:3;
>>>   	unsigned	rx_nbits:3;
>>> -- 
>>> 2.7.4
>>>