linux-kernel - Re: [PATCH 2/2 v2] SPI: spi-pxa2xx: SPI support for Intel Quark X1000

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Date:	Wed, 08 Oct 2014 10:02:07 +0100
From:	Bryan O'Donoghue <pure.logic@...us-software.ie>
To:	"Chen, Alvin" <alvin.chen@...el.com>,
	Eric Miao <eric.y.miao@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Mark Brown <broonie@...nel.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Westerberg, Mika" <mika.westerberg@...el.com>,
	"Kweh, Hock Leong" <hock.leong.kweh@...el.com>,
	"Ong, Boon Leong" <boon.leong.ong@...el.com>,
	"Tan, Raymond" <raymond.tan@...el.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH 2/2 v2] SPI: spi-pxa2xx: SPI support for Intel Quark X1000

On 08/10/14 08:48, Chen, Alvin wrote:

> Now, we have another board which can support 4 slave spi per master, but not only Galileo. Since that board is not public, after discussing with team, we decide to make the
> upstream code to support '1'.
>
> I will change it back to
> .num_chipselect = 1,

Hi Alvin.

The important thing in terms of Galileo is to ensure that a GPIO can be 
used for chip-select.

The user-space API ported from Arduino to Linux wants to control it's 
own chip-select directly - so the internal chip-select of the Quark SPI 
master can - and does de-assert while doing SPI transactions on Galileo. 
The CS on the master is tied to FIFO occupancy - so at higher bit-rates 
we can fail to keep the FIFO occupied :(

That doesn't matter though, because the pinned out SPI:CS on the Arduino 
header is a GPIO.

 From the perspective of the Arduino code in user-space and the slave 
hardware @ the other end of the SPI bus - we see a nice and consistent 
chip-select for the entire duration of the SPI transaction - even though 
the actual SPI:CS coming from the SoC can *waggle* - when FIFOs go empty.

IMO - so long as you've tested on Galileo and seen working SPI - you're 
good to go anyway.

Bryan
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