linux-kernel - Re: [PATCH 1/2] spi: spi-omap2-mcspi.c: Add dts for slave device configuration.

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Date:	Thu, 6 Jun 2013 13:08:46 +0300
From:	Illia Smyrnov <x0194613@...com>
To:	Mark Brown <broonie@...nel.org>
CC:	Illia Smyrnov <illia.smyrnov@...com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>, Daniel Mack <zonque@...il.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Tony Lindgren <tony@...mide.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<devicetree-discuss@...ts.ozlabs.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<spi-devel-general@...ts.sourceforge.net>
Subject: Re: [PATCH 1/2] spi: spi-omap2-mcspi.c: Add dts for slave device
 configuration.

On 06/05/2013 02:57 PM, Mark Brown wrote:
> On Wed, Jun 05, 2013 at 02:39:57PM +0300, Illia Smyrnov wrote:
>
>> +SPI Controller specific data in SPI slave nodes:
>> +- The spi slave nodes can provide the following information which is used
>> +  by the spi controller:
>> +  - ti,spi-turbo-mode: Set turbo mode for this device.
>> +
>
> What is turb mode and

According to OMAP TRM [1] MCSPI turbo mode improves the throughput of 
the SPI interface when a single channel is enabled by allowing transfers 
until the shift register and the MCSPI_RXx register are full.

I tested turbo mode using KS8851 SPI Ethernet controller on Blaze with 
OMAP4460 and nuttcp tool with -r for RX throughput measuring. Enabling 
turbo mode was increased throughput form 7.5538 Mbps to 8.3848 Mbps

> why would we not want to just enable it all the
> time?

Turbo mode gives the expected results not for all cases. There are some 
limitations:
- works only if a single channel is enabled (no effect when several 
channels are enable);
- improves the throughput on RX direction only;
- effective only when a transfer exceeds two words. For single SPI
word transfers OMAP TRM [1] recommends deactivate turbo mode.

So it is useful to have the property in DT, that allow us to switch 
turbo mode off/on for certain slave.

> Based on this documentation it's not really possible to tell...
>
I will add turbo mode description to documentation in the next patch version

[1]: http://www.ti.com/lit/ug/swpu235z/swpu235z.pdf
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