Date:   Mon, 26 Nov 2018 14:20:27 +0000
From:   Vokáč Michal <Michal.Vokac@...ft.com>
To:     Rob Herring <robh@...nel.org>
CC:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Jyri Sarha <jsarha@...com>
Subject: Re: [PATCH fbdev-for-next 2/2] video: ssd1307fb: Add support for the
 reset-active-low property

On 26.11.2018 14:49, Rob Herring wrote:
> On Mon, Nov 26, 2018 at 6:25 AM Vokáč Michal <Michal.Vokac@...ft.com> wrote:
>>
>> On 19.11.2018 23:32, Rob Herring wrote:
>>> On Mon, Nov 19, 2018 at 9:12 AM Vokáč Michal <Michal.Vokac@...ft.com> wrote:
>>>> On 12.11.2018 17:55, Rob Herring wrote:
>>>>> On Fri, Nov 02, 2018 at 02:56:35PM +0000, Vokáč Michal wrote:
>>>>>> The SSD130x OLED display reset signal is active low. Now the reset
>>>>>> sequence is implemented in such a way that DTS authors are forced to
>>>>>> define the reset-gpios property with GPIO_ACTIVE_HIGH to make the reset
>>>>>> work.
>>>>>>
>>>>>> Add the reset-active-low property so the signal is inverted once again
>>>>>> and the GPIO_ACTIVE_LOW work as expected.
>>>>>>
>>>>>> Signed-off-by: Michal Vokáč <michal.vokac@...ft.com>
>>>>>> ---
>>>>>>     drivers/video/fbdev/ssd1307fb.c | 6 ++++--
>>>>>>     1 file changed, 4 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/video/fbdev/ssd1307fb.c b/drivers/video/fbdev/ssd1307fb.c
>>>>>> index e7ae135..790f1c4 100644
>>>>>> --- a/drivers/video/fbdev/ssd1307fb.c
>>>>>> +++ b/drivers/video/fbdev/ssd1307fb.c
>>>>>> @@ -608,6 +608,7 @@ static int ssd1307fb_probe(struct i2c_client *client,
>>>>>>        struct fb_deferred_io *ssd1307fb_defio;
>>>>>>        u32 vmem_size;
>>>>>>        struct ssd1307fb_par *par;
>>>>>> +    bool reset_active_low;
>>>>>>        u8 *vmem;
>>>>>>        int ret;
>>>>>>
>>>>>> @@ -671,6 +672,7 @@ static int ssd1307fb_probe(struct i2c_client *client,
>>>>>>        par->com_seq = of_property_read_bool(node, "solomon,com-seq");
>>>>>>        par->com_lrremap = of_property_read_bool(node, "solomon,com-lrremap");
>>>>>>        par->com_invdir = of_property_read_bool(node, "solomon,com-invdir");
>>>>>> +    reset_active_low = of_property_read_bool(node, "reset-active-low");
>>>>>>
>>>>>>        par->contrast = 127;
>>>>>>        par->vcomh = par->device_info->default_vcomh;
>>>>>> @@ -728,9 +730,9 @@ static int ssd1307fb_probe(struct i2c_client *client,
>>>>>>
>>>>>>        if (par->reset) {
>>>>>>                /* Reset the screen */
>>>>>> -            gpiod_set_value_cansleep(par->reset, 0);
>>>>>> +            gpiod_set_value_cansleep(par->reset, reset_active_low);
>>>>>>                udelay(4);
>>>>>> -            gpiod_set_value_cansleep(par->reset, 1);
>>>>>> +            gpiod_set_value_cansleep(par->reset, !reset_active_low);
>>>>>
>>>>> I think you and whomever wrote the original code are misinterpretting
>>>>> how the gpiod API works. 1 means make the signal active and this case
>>>>> active is low.
>>>>
>>>> I totally agree and I think I understand that correctly.
>>>>
>>>>> It is strange, but does mean a reset sequence should always be set to a
>>>>> 1 and then a 0 and it will work with either polarity in the DT.
>>>>
>>>> I agree the reset should be done as a 0 -> 1 -> 0 sequence and that should
>>>> just work. The problem is it is implemented vice versa and so it works only
>>>> if you have GPIO_ACTIVE_HIGH in DT for a signal that is actually active low.
>>>>
>>>> And what it actually does is that it holds the controller in reset since
>>>> the GPIO is successfully acquired (because of GPIOD_OUT_LOW here [1]) and
>>>> later on it only releases the reset.
>>>>
>>>> As a DT author I would like to somehow clearly state that the OLED display
>>>> uses active low reset in my DT.
>>>>
>>>> My first attempt to fix this was to change the reset sequence [2].
>>>> It was applied and then reverted as it is not backward compatible with
>>>> deployed DTB files [3].
>>>>
>>>> [1] https://elixir.bootlin.com/linux/v4.20-rc3/source/drivers/video/fbdev/ssd1307fb.c#L570
>>>> [2] https://patchwork.kernel.org/patch/10617729/
>>>> [3] https://patchwork.kernel.org/patch/10617731/
>>>
>>> Okay, now I understand the background. We've hit this somewhere else too.
>>>
>>> Rather than have a binding demonstrating what not to do, I'd like to
>>> fix this in another way. I also don't want to live with this forever
>>> when there's only 1 board affected (in tree at least) and there's only
>>> an ABI if someone notices (I'm happy though that the maintainers
>>> caught this). There's 2 other options. The 1st is add a fixup to the
>>> DT for this platform to ensure that the GPIO is configured active low
>>> (the Versatile platform code has an example fixup). With that, apply
>>> what was originally applied (or revert the revert). The fixup could be
>>> applied only after someone complains their display broke. The 2nd
>>> option is just add an of_machine_is_compatible check within this
>>> driver. In that case, you wouldn't fix dts file for the platform
>>> (unless you also want to manually check reset-gpios).
>>
>> Hi Rob,
>>
>> I am still trying to figure out what exactly you meant by the 1st and
>> 2nd option. Both concepts are new to me.
>>
>> Regarding the 1st option, what you meant by "this platform" here:
>>> Add a fixup to the DT for this platform
>> The only board in tree that uses the OLED (imx28-cfa10036) and its
>> dts file?
> 
> Yes, that one.
> 
>> I am also not sure where to look for the example. When you say
>> Versatile platform code I tend to look into plat-versatile or
>> mach-versatile. I could not find anything I could use as an example
>> in there. I think that is not what you meant.
> 
> See versatile_dt_pci_init(). Or look for other callers of of_update_property().

Excellent, I will look at that.
  
>> Regarding the 2nd option, you suggest to use of_machine_is_compatible
>> to decide what reset sequence to use? In case of imx28-cfa10036 use
>> the old 0 -> 1, in all other cases use a new correct sequence 1 -> 0?
>> That also does not seem right.
> 
> Correct. Though if you fix imx28-cfa10036 dts, then you have to handle
> that case too.
> 
> Why is it not right? Ugly yes, but it's not wrong.

Ugly is what I probably meant. It seems like other users (among drivers)
of of_machine_is_compatible are mostly drivers for IP blocks that need
slightly different handling on different SoC variants.

Thank you very much,
Michal

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