linux-kernel - Re: [RFC 2/2] mux: mmio-based syscon mux controller

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Date:   Wed, 19 Apr 2017 09:23:34 -0700
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Philipp Zabel <p.zabel@...gutronix.de>,
        Peter Rosin <peda@...ntia.se>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sakari Ailus <sakari.ailus@....fi>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [RFC 2/2] mux: mmio-based syscon mux controller



On 04/19/2017 08:27 AM, Philipp Zabel wrote:
> On Wed, 2017-04-19 at 13:58 +0200, Peter Rosin wrote:
>> On 2017-04-19 13:50, Philipp Zabel wrote:
>>> On Thu, 2017-04-13 at 18:09 -0700, Steve Longerbeam wrote:
>>>>
>>>> On 04/13/2017 08:48 AM, Philipp Zabel wrote:
>>>>> This adds a driver for mmio-based syscon multiplexers controlled by a
>>>>> single bitfield in a syscon register range.
>>>>>
>>>>> Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
>>>>> ---
>>>>>   drivers/mux/Kconfig      |  13 +++++
>>>>>   drivers/mux/Makefile     |   1 +
>>>>>   drivers/mux/mux-syscon.c | 130 +++++++++++++++++++++++++++++++++++++++++++++++
>>>>>   3 files changed, 144 insertions(+)
>>>>>   create mode 100644 drivers/mux/mux-syscon.c
>>>>>
>>>>> diff --git a/drivers/mux/Kconfig b/drivers/mux/Kconfig
>>>>> index 86668b4d2fc52..a5e6a3b01ac24 100644
>>>>> --- a/drivers/mux/Kconfig
>>>>> +++ b/drivers/mux/Kconfig
>>>>> @@ -43,4 +43,17 @@ config MUX_GPIO
>>>>>   	  To compile the driver as a module, choose M here: the module will
>>>>>   	  be called mux-gpio.
>>>>>
>>>>> +config MUX_SYSCON
>>>>
>>>> my preference would be CONFIG_MUX_MMIO.
>>>>
>>>>> +	tristate "MMIO bitfield-controlled Multiplexer"
>>>>
>>>> "MMIO register bitfield-controlled Multiplexer"
>>>>
>>>> The rest looks good to me.
>>>
>>> I'll change those. mux-syscon.c should probably be renamed to
>>> mux-mmio.c, too.
>>
>> I think I disagree. But I'm not familiar with syscon so I don't know.
>> IIUC, syscon uses regmap to do mmio and this driver requires syscon
>> to get at the regmap, and in the end this driver doesn't know anything
>> about mmio. All it knows is syscon/regmap.
>
> That is a good point. Right now there is nothing MMIO about the driver
> except for the hardware that I want it to handle.
>
>>  If some warped syscon
>> thing shows up that wraps something other than mmio in its regmap,
>> this driver wouldn't care about it. And syscon is something that
>> is also known in the DT world. Given that, I think everything in this
>> driver should be named syscon and not mmio.
>>

My argument against using the name "syscon" in the device tree is that
it is referring to a subsystem in the Linux kernel. Besides the fact
that "syscon" does not clearly describe, at least to me, what sort of
device this mux is. "regmap" also has similar problem in that it refers
to  a Linux subsystem, although it is clearer to me at least, what the
mux is composed of (a mapped register). Personally I still like mmio,
most embedded systems access registers via MMIO, and the name is not
referring to any specific Linux subsystem.

Steve




>> Or?
>
> Well, ...
>
> the driver could be extended to do actual MMIO if the syscon is not
> found. This would work only if it has exclusive access to its register.
>
> On the other hand, the driver could also be made to match against
>     compatible = "bitfield-mux",
> for example, and allow handling muxes inside SPI or I2C controlled MFD
> devices that provide a syscon regmap, as you describe:
>
> 	spi-host {
> 		mfd-device {
> 			compatible = "some-spi-regmap-device";
>
> 			mux {
> 				compatible = "bitfield-mux";
> 			};
> 		};
> 	};
>
> regards
> Philipp
>