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

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Message-ID: <CAMo8Bf+mC_ZfECqJ2Apf5rcTJ10hbE55wN1cT7Jv74kzWFuVTg@mail.gmail.com>
Date:	Wed, 12 Mar 2014 21:13:35 +0400
From:	Max Filippov <jcmvbkbc@...il.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	"linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>,
	linux-spi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	"devicetree@...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 4:49 PM, Mark Brown <broonie@...nel.org> wrote:
> 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).

Looks like this swap is not needed: I've looked at drivers that set
bits_per_word = 16, they submit data in cpu endianness. Will fix in v3.

-- 
Thanks.
-- Max
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