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

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Date:   Mon, 28 Jan 2019 22:28:14 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Baolin Wang <baolin.wang@...aro.org>,
        lanqing.liu@...eadtrum.com
Subject: Re: [PATCH v3 1/2] spi: support inter-word delay requirement for
 devices

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?

Thanks,
Jonas