linux-kernel - Re: [PATCH] spi: xilinx: Use standard num-cs binding

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Message-ID: <74214fc9-e5e4-42db-bd85-51ed02975388@BN1AFFO11FD026.protection.gbl>
Date:	Tue, 31 Mar 2015 10:16:57 +0200
From:	Michal Simek <michal.simek@...inx.com>
To:	Mark Brown <broonie@...nel.org>,
	Michal Simek <michal.simek@...inx.com>
CC:	<linux-kernel@...r.kernel.org>, <monstr@...str.eu>,
	Sören Brinkmann 
	<soren.brinkmann@...inx.com>, <linux-spi@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] spi: xilinx: Use standard num-cs binding

On 03/31/2015 07:47 AM, Mark Brown wrote:
> On Mon, Mar 30, 2015 at 08:46:10AM +0200, Michal Simek wrote:
>> On 03/27/2015 06:53 PM, Mark Brown wrote:
>>> On Fri, Mar 27, 2015 at 11:55:49AM +0100, Michal Simek wrote:
> 
>>> Please fix your mail client to word wrap within paragraphs at less than
>>> 80 columns - this makes your mails easier to read and reply to.
> 
>> You are the first one who had problem with this. But I have setup lower
>> limit and hopefully it is better now.
> 
> That looks better, yes...  I may just be the first one who's bothered
> remarking on this.

yes and I definitely thank you for that.

>>>> On 03/08/2015 08:00 PM, Mark Brown wrote:
>>>>> On Fri, Jan 16, 2015 at 01:55:14PM +0100, Michal Simek wrote:
> 
>>> Remember that we can at least in theory have additional chip selects
>>> that aren't controlled by the IP block but are instead GPIOs.  
> 
>> I agree with you but this can be generic case for every SPI driver. Also
>> using external decoder is possible for every driver. Maybe there are
>> others options via I2C too.
> 
> Remember that this in the context of me saying I don't think num-cs is
> a particularly good idea at all...

yes.


>>> There's
>>> also some potential confusion for users between the number of chip
>>> selects in use in a given system and the size of the bitfield that the
>>> driver needs to take care of.
> 
>> num-ss-bits is autogenerated directly from design tools for particular
>> hardware design and this size is exactly setup and hardcoded. (num-cs
>> can be just the same case)
>> If there are 5 bits there are 5 wires from IP. And value of num-ss-bits
>> and num-cs will be the same.
> 
> But what your patch did was *replace* num-ss-bits in the binding, not
> just add it.

yes. Sync binding was the main my point.


>> If user wants to use less lines then physically available we could
>> potentially extend binding to say. num-ss-bit - number of chip selects
>> available in hardware. num-cs - number of chip selects used by the driver.
>> But I expect that this will be rejected because it is software setting
>> not hardware description.
> 
> num-cs *is* a software setting.

ok - what to do with that? Remove it because it shouldn't be passed via DT?

Thanks,
Michal

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