linux-kernel - Re: [PATCH 1/3 v5] usb: phy-samsung-usb: Simplify PMU register handling

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Date:	Wed, 07 Aug 2013 20:50:21 +0200
From:	Sylwester Nawrocki <s.nawrocki@...sung.com>
To:	Julius Werner <jwerner@...omium.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Pawel Moll <Pawel.Moll@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Felipe Balbi <balbi@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Tomasz Figa <t.figa@...sung.com>,
	Vivek Gautam <gautam.vivek@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Sylwester Nawrocki <sylvester.nawrocki@...il.com>
Subject: Re: [PATCH 1/3 v5] usb: phy-samsung-usb: Simplify PMU register handling

On 08/07/2013 07:06 PM, Julius Werner wrote:
>> This breaks compatibility, both for an old kernel and a new dt and a new
>> kernel with an old dt. Is anyone using these bindings?
> 
> They only affect Samsung SoCs and have only been upstream for half a
> year, so I doubt it's heavily used.

It probably wouldn't be of much concern, but I can't tell for sure.

>> Why are we describing fewer registers now? Are they described elsewhere?
>>
>> The dt should describe the device, not only the portion of it Linux
>> wants to use right now.
> 
> This only ever described a small section of the huge set of PMU
> registers anyway. 

The PMU registers are already scattered across various drivers, like
Power Domain, various PHYs (USB/HSIC/MIPI CSI-2/DSI, ADC, ...), Reset
and more. And it happens currently there is no central driver for
the Power Management Unit, that would, for example expose the regmap
interface that the interested drivers could use. The downside of this
would be that each, e.g. USB PHY driver would need to know SoC specific
offsets to its registers in the PMU.

Before device tree started to be used those PMU registers were controlled
by respective drivers, mostly through platform_data callbacks. As they
could be considered parts of given device.

                   Before it described up to three registers
> controlling different PHYs (using hardcoded offsets in the code to
> later find the right one)... with my patch every PHY's DT entry only
> describes the one register concerning itself, which makes more sense
> in my opinion. It will also prevent the register descriptions in
> different DT entries from overlapping.

Yes, the patch looks sensible. Since currently each USB PHY has its
own device tree node it looks wrong to have the reg property in the
usbphy-sys subnodes defining register region for all possible PHYs.
And we indeed and up with multiple reg properties pointing to same
register region.

However the biggest drawback is breaking backwards compatibility.
I'm not sure if those changes are worth it, especially that all those
USB PHY drivers are supposed to be rewritten to use the generic PHY API.

Thanks,
Sylwester
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