Date:   Mon, 30 Jan 2023 12:37:06 -0600
From:   Rob Herring <robh@kernel.org>
To:     "Leonard, Niall" <Niall.Leonard@ncr.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] gpio: dt-bindings: add new property to wd,mbl-gpio
 bindings
Message-ID: <20230130183706.GA3079628-robh@kernel.org>
References: <20230126-gpio-mmio-fix-v1-0-8a20ce0e8275@ncr.com>
 <20230126-gpio-mmio-fix-v1-1-8a20ce0e8275@ncr.com>
 <d4a70ab2-c5a2-2478-3fa9-c703c1619252@linaro.org>
 <MW5PR15MB52184F83BA524D9600B820F5FDCC9@MW5PR15MB5218.namprd15.prod.outlook.com>
 <ed16faa2-eb04-772d-8762-0c3f90fddbcb@linaro.org>
 <c95cea18-5b92-e16c-f4f6-cde93ca22ca1@ncr.com>
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In-Reply-To: <c95cea18-5b92-e16c-f4f6-cde93ca22ca1@ncr.com>
Precedence: bulk

On Mon, Jan 30, 2023 at 01:20:55PM +0000, Leonard, Niall wrote:
> On 29/01/2023 15:59, Krzysztof Kozlowski wrote:
> > *External Message* - Use caution before opening links or attachments
> > 
> > On 27/01/2023 12:39, Leonard, Niall wrote:
> >>
> >>
> >>> -----Original Message-----
> >>> From: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> >>> Sent: 26 January 2023 12:29
> >>> To: Leonard, Niall <Niall.Leonard@ncr.com>; Linus Walleij
> >>> <linus.walleij@linaro.org>; Bartosz Golaszewski <brgl@bgdev.pl>; Rob
> >>> Herring <robh+dt@kernel.org>; Krzysztof Kozlowski
> >>> <krzysztof.kozlowski+dt@linaro.org>
> >>> Cc: linux-gpio@vger.kernel.org; devicetree@vger.kernel.org; linux-
> >>> kernel@vger.kernel.org
> >>> Subject: Re: [PATCH 1/3] gpio: dt-bindings: add new property to wd,mbl-gpio
> >>> bindings
> >>>
> >>> *External Message* - Use caution before opening links or attachments
> >>>
> >>> On 26/01/2023 11:17, Niall Leonard via B4 Submission Endpoint wrote:
> >>>> From: Niall Leonard <nl250060@ncr.com>
> >>>
> >>> Subject: missing "wd,mbl-gpio:" prefix.
> >>>
> >>> Subject: drop second/last, redundant "bindings". The "dt-bindings"
> >>> prefix is already stating that these are bindings.
> >>>
> >>>>
> >>>> Added optional "no-input" property
> >>>
> >>> Missing full stop.
> >>>
> >>>>
> >>>> Signed-off-by: Niall Leonard <nl250060@ncr.com>
> >>>> ---
> >>>>   Documentation/devicetree/bindings/gpio/wd,mbl-gpio.txt | 1 +
> >>>>   1 file changed, 1 insertion(+)
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/gpio/wd,mbl-gpio.txt
> >>>> b/Documentation/devicetree/bindings/gpio/wd,mbl-gpio.txt
> >>>> index 038c3a6a1f4d..9405f9dad522 100644
> >>>> --- a/Documentation/devicetree/bindings/gpio/wd,mbl-gpio.txt
> >>>> +++ b/Documentation/devicetree/bindings/gpio/wd,mbl-gpio.txt
> >>>> @@ -18,6 +18,7 @@ Required properties:
> >>>>
> >>>>   Optional properties:
> >>>>   	- no-output: GPIOs are read-only.
> >>>> +	- no-input: GPIOs are write-only. Read is via a shadow register.
> >>>
> >>> Why this property is needed? Why driver cannot always use shadow
> >>> register?
> >>>
> >> The shadow register is currently only used during the write operation. It is not available during the read operation.
> > 
> > You just wrote above that reading is via shadow register, so how can it
> > not be available for reads? Again, why you cannot always read via shadow
> > register and need to make a property? You mean that for other GPIOs
> > there is no shadow register at all?
> > 
> The existing read method does not use the shadow register.
> 
> static int bgpio_get(struct gpio_chip *gc, unsigned int gpio)
> {
> 	return !!(gc->read_reg(gc->reg_dat) & bgpio_line2mask(gc, gpio));
> }
> 
> > What changes between one board and another that justifies this property?
> 
> I have a couple of boards where the electronics engineer decided to only 
> use the chip select line, so no read/write signal is connected. This 
> means that reading the address activates the chip select and drives the 
> contents of the data bus to the port. 

This part makes sense as you explained the h/w.

> For example is someone reads the 
> file /sys/kernel/debug/gpio this corrupts the port. So I have had to add 
> this property to avoid that situation.

Not quite relevant to the DT binding being a Linux detail.

> 
> If you are strongly against this then just reject it and I will look 
> after it myself. I thought there may be others who would find this 
> change useful.

A property for a board level quirk is appropriate. You just need to 
explain that in the commit message rather than stating what the diff 
already tells us.

Rob