linux-kernel - Re: [PATCH v4 2/2] spi: add driver for J-Core SPI controller

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Date:	Tue, 2 Aug 2016 13:11:36 -0500
From:	Rob Landley <rob@...dley.net>
To:	Rich Felker <dalias@...c.org>, Mark Brown <broonie@...nel.org>
Cc:	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-spi@...r.kernel.org, linux-sh@...r.kernel.org,
	j-core@...ore.org, Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH v4 2/2] spi: add driver for J-Core SPI controller

On 08/01/2016 08:45 PM, Rich Felker wrote:
> Yes. 50 MHz is the natural default frequency, but I found out at the
> last minute from the hardware engineers that clocking the current SoC
> up to 62.5 MHz (for faster cpu) will require the SPI timing to be
> programmed based on the faster reference clock. This messes up the
> ability to get optimal SD card transfer rates, so we'll probably end
> up having a real 50 MHz clock for the SPI anyway, but I thought it was
> important to be able to handle this issue in the DT binding anyway.

For those of you following along at home, the first open source VHDL
release of http://j-core.org runs at 33 mhz on Spartan 6 FPGAs, the
second at 50mhz on the same hardware, and you'd think the third would be
66mhz as the chip continues to be optimized, but one of the new I/O
busses requires a multiple of 12.5 mhz and they didn't want to add a
clock domain crossing thing for it, hence 62.5 mhz.

Earlier it was easy to get everything to work off a single master clock,
but as we add more peripherals to the SOC and speed things up stuff's
likely to change. (Development is ongoing, and we have a public
http://j-core.org/roadmap.html out through 2019. It you're waiting for
stuff to stop changing before getting basic board support in that runs
on existing releases, it's might be a while.)

Rob