linux-kernel - Re: [PATCH 3/3] add support for DWC UFS Host Controller

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Date:	Mon, 8 Feb 2016 15:36:52 +0000
From:	Joao Pinto <Joao.Pinto@...opsys.com>
To:	Mark Rutland <mark.rutland@....com>,
	Joao Pinto <Joao.Pinto@...opsys.com>
CC:	Arnd Bergmann <arnd@...db.de>, <santosh.sy@...sung.com>,
	<h.vinayak@...sung.com>, <julian.calaby@...il.com>,
	<akinobu.mita@...il.com>, <hch@...radead.org>,
	<gbroner@...eaurora.org>, <subhashj@...eaurora.org>,
	<CARLOS.PALMINHA@...opsys.com>, <ijc+devicetree@...lion.org.uk>,
	<linux-kernel@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH 3/3] add support for DWC UFS Host Controller

Hi Mark,

On 2/8/2016 3:30 PM, Mark Rutland wrote:
> On Mon, Feb 08, 2016 at 03:17:11PM +0000, Joao Pinto wrote:
>> Hi Mark and Arnd,
>>
>> I am planning the v2 of this patch set. I have a doubt in the version
>> compatibility strings... The core driver must support the UFS 2.0 controller and
>> this patch set includes a patch that adds 2.0 capabilities to it.
> 
> Ok. It wasn't clear to me that this series added support for features
> specific to 2.0.

Yes, the patch set contains a patch to add 2.0 to the UFS core driver.
The cover letter:
https://lkml.org/lkml/2016/2/3/331
The Patch:
https://lkml.org/lkml/2016/2/3/330

> 
>> The core driver can get from the controller's version and with that
>> use or not a specific 2.0 feature.
> 
> It can be detected from the hardware?

Yes, the hardware has a register that contains the version, and so if a driver
has workarounds then it can adapt.

> 
>> What would be the real added-value of having a compatibility string like
>> "snps,ufshcd-1.1" and "snps,ufshcd-2.0" if the driver can perform as 2.0 if it
>> detects a 2.0 controller?
> 
> Generally having specify strings ensure that it's possible to handle
> things in future (e.g. errata workarounds), or if we realise something
> isn't as clear-cut as we thought it was (i.e. 2.0 not being a strict
> superset of 1.1).
> 



> It's difficult to predict when you need that, so we err on the side of
> requiring it. At worst it means you have a small redundant few
> characters in a DT, but that's a much better proposition than having too
> little information.
> 
>> Are you saying that a user that puts "snps,ufshcd-1.1"
>> in the DT compatibility string disables the UFS 2.0 in the core driver despite
>> the controller is 2.0? Please clarify.
> 
> If you can consistently and safely detect that the HW is 2.0, using 2.0
> functionality is fine.
> 
> Regardless, you should have a -1.1 compatible string for the 1.1 HW, and
> a -2.0 string for the 2.0 HW, so that DTs are explicit about what the
> hardware is. If 2.0 is intended to be a superset of 1.1, you can have a
> 1.1 fallback entry for the 2.0 hardware.
> 

Ok, I will include the version in the compatibility strings, but if someone
mentions "snps,ufshcd-1.1" only and the driver detects that the HW is 2.0
capable it will activate the 2.0 features independently of what mentioned in the
DT, correct?

> Thanks,
> Mark.
> 

Thanks,
Joao