Re: [PATCH v2 1/3] spi: Introduce one new field to set word delay

From: Jan Kundrát
Date: Wed Aug 29 2018 - 07:33:30 EST

On Ätvrtek 16. srpna 2018 14:54:49 CEST, Baolin Wang wrote:
+ * @word_delay: clock cycles to inter word delay after each word size
+ * (set by bits_per_word) transmission.

I need a similar functionality for talking to a SPI device from userspace -- see my attempt for implementing this in spi-orion.c at [1]. The device's datasheet says that I should wait, e.g., 3Âs between each two words. I therefore like this patch :).

The description can be improved because it left me wondering what "clock this is about. I suppose it's about the SPI clock cycles and not CPU clock cycles, right? I'll be hapy to patch this once Baolin confirms that that is the intended meaning.

It seems that this is only implemented in one newly added driver. I'm interested in supporting this in spi-orion.c, but that sounds like driver-specific work for something which is pretty generic. How should this be implemented? Given that drivers for SPI masters can implement a function which transfers several words at once, there are not that many better possibilities than adding udelay()s, though. Thoughts?

What is your plan to do with drivers which do not implement this (yet)? If a spi_transfer gets queued which asks for a word_delay delay, it is silently ignored now, AFAIU.

What about userspace support, spidev and spi_ioc_transfer (that's my target, actually)? Is it OK to s/pad/word_delay/ in the spidev code and pass that to the generated struct spi_transfer? In my opinion, once we support specifying this from userspace, one has to definitely check that the SPI controller is ready to honor this request. Do we want a new bit in spi_controller.flags for this?

With kind regards,