linux-kernel - Re: [PATCH v2 1/3] spi: add xtfpga SPI controller driver

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Message-ID: <20140312124901.GQ28112@sirena.org.uk>
Date:	Wed, 12 Mar 2014 12:49:01 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Max Filippov <jcmvbkbc@...il.com>
Cc:	linux-xtensa@...ux-xtensa.org, linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	Chris Zankel <chris@...kel.net>,
	Marc Gauthier <marc@...ence.com>,
	Rob Herring <robh+dt@...nel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 1/3] spi: add xtfpga SPI controller driver

On Wed, Mar 12, 2014 at 04:21:03PM +0400, Max Filippov wrote:

> +	case 16:
> +		/* Bytes that should go out earlier have lower addresses,
> +		 * but the hardware operates with 16 bit words and transmits
> +		 * higher bits first. Thus data in memory is in BE order.
> +		 */
> +		xspi->data = (xspi->data << 16) | be16_to_cpu(v);
> +		xspi->data_sz += 2;
> +		break;

Are you sure you need to do this byte swap?  SPI words are big endian so
if you are running on a little endian processor if you are transmitting
16 bit words you should see the data getting swapped on the bus (this is
why I say you should never have more than 8 bits per word for regmap).

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