linux-kernel - Re: [PATCH 2/2] spi: bcm-mspi: Add support for Broadcom MSPI driver.

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Date:	Thu, 14 May 2015 11:19:01 -0700
From:	Scott Branden <sbranden@...adcom.com>
To:	Mark Brown <broonie@...nel.org>
CC:	Jonathan Richardson <jonathar@...adcom.com>,
	Dmitry Torokhov <dtor@...gle.com>,
	Anatol Pomazau <anatol@...gle.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	"Kumar Gala" <galak@...eaurora.org>,
	<linux-kernel@...r.kernel.org>, <linux-spi@...r.kernel.org>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH 2/2] spi: bcm-mspi: Add support for Broadcom MSPI driver.

Hi Mark,

On 15-05-14 03:31 AM, Mark Brown wrote:
> On Wed, May 13, 2015 at 05:19:06PM -0700, Scott Branden wrote:
>
>> The purpose of this mspi interface is to connect to NOR flash.  There are
>> other SPI interfaces on the devices used to connect to other SPI devies.  We
>> don't have any need to support full duplex slaves on this port (NOR have any
>> hardware wired this way - pun realized after typing this).
>
> Chip vendors often say this sort of thing and then get surprised by what
> their users choose to do, and even if it only ever gets used with flash
> all it would take is some new flash command which can use full duplex
> for something.  Please write the code so it at least tries to handle
> full duplex operation, if you can't test it fully that's not the end of
> the world.  It doesn't look like it should be particularly difficult.
>
Yes, there is always room for improvements in code.  In this case - it 
really is not worth our time to add code we can't test.  We try to 
deliver code that we can test and actually works.  Yes, if anyone needs 
to use the mspi for full duplex operation code can be added in the 
future - it is software.  This block has gone through many generations 
of our SoCs and has only been used for this purpose - the bootROM boots 
from this SPI only.  It is dedicated for this purpose.

Also, there are other SPI blocks on the SoC that are used for connect 
other SPI devices.  These SPI blocks are properly designed for 
full-duplex operation.  This mspi block is designed for connecting to 
NOR flash.

Regards,
Scott



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