linux-kernel - Re: [PATCH v3 1/2] spi: support inter-word delay requirement for devices

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Date:   Tue, 29 Jan 2019 10:14:19 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Mark Brown <broonie@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Lanqing Liu <lanqing.liu@...eadtrum.com>
Subject: Re: [PATCH v3 1/2] spi: support inter-word delay requirement for
 devices



On 29/01/2019 10:04, Baolin Wang wrote:
> Hi Jonas,
> On Tue, 29 Jan 2019 at 05:28, Jonas Bonn <jonas@...rbonn.se> wrote:
>>
>> Hi,
>>
>> On 28/01/2019 19:10, Mark Brown wrote:
>>> On Sat, Jan 26, 2019 at 05:32:19PM +0100, Jonas Bonn wrote:
>>>
>>>> @@ -164,6 +166,7 @@ struct spi_device {
>>>>       char                    modalias[SPI_NAME_SIZE];
>>>>       const char              *driver_override;
>>>>       int                     cs_gpio;        /* chip select gpio */
>>>> +    uint16_t                word_delay;     /* inter-word delay (us) */
>>>
>>> This needs some code in the core joining it up with the per-transfer
>>> word delay similar to what we have for speed_hz and bits_per_word in
>>> __spi_validate().  Then the controller drivers can just look at the
>>> per-transfer value and support both without having to duplicate logic.
>>>
>>
>> So spi_transfer already has a field word_delay and it's defined as
>> _clock cycles_ to delay between words.  I defined word_delay in
>> spi_device as _microseconds_ to delay along the lines of delay_usecs.
>>
>> Given that the inter-word delay is a function of the slave device speed
>> and not of the SPI bus speed, I'm inclined to say that a time-based
>> delay is what we want (to be independent of bus speed).  As such, I want
>> to know if I should add word_delay_usecs to _both_ spi_transfer and
>> spi_device?
>>
>> There's only one user of word_delay from spi_transfer.  Just looking at
>> it quickly, it looks like it wants the word_delay in
>> SPI-controller-clock cycles and not SCK cycles which seems pretty broken
>> to me.  Adding Baolin and Lanqing to CC: for comment.  Could we rework
>> that to be microseconds and do the calculation in the driver?
> 
> The Spreadtrum SPI controller's word delay unit is clock cycle of the
> SPI clock, since the SPI source clock can be changed, we can not force
> user to know the real microseconds. But can we change it to a union
> structure? not sure if this is a good way.

OK, so it is the SPI clock.  That's good.  There's a comment in the 
driver that makes it look like it should be the source clock.

The problem with a delay in clock cycles is that the faster the clock, 
the shorter the delay.  The delay is a property of the slave and the 
slave has a fixed internal clock.  This means that if we increase SCK we 
also need to increase the word_delay (in cycles) in order to give the 
slave the same amount of breathing room.

> 
> union {
>       int word_delay_us;
>       int word_delay_cycle;
> } w;
> 

I don't think that's a practical solution.

The register setting in the spi-sprd driver is what... SCK cycles?  So 
you'd want word_delay_us * max_speed_hz?

The register setting on my Atmel board is in SPI-clock cycles 
(effectively).  So I want word_delay_us*clk_get_rate(spi-clk).

/Jonas