Subject: Re: [PATCH] leds: leds-pca963x: workaround group blink scaling issue
To: Matt Ranostay <mranostay@gmail.com>, Tony Lindgren <tony@atomide.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Matt Ranostay <matt@ranostay.consulting>,
        Rob Herring <robh+dt@kernel.org>, Mark Rutland <Mark.Rutland@arm.com>
From: Jacek Anaszewski <j.anaszewski@samsung.com>
Message-id: <5d9476b8-b552-f745-e06d-9894fa2e542a@samsung.com>
Date: Mon, 17 Oct 2016 09:58:26 +0200
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On 10/15/2016 02:00 PM, Matt Ranostay wrote:
> On Fri, Oct 14, 2016 at 7:20 AM, Tony Lindgren <tony@atomide.com> wrote:
>> * Jacek Anaszewski <j.anaszewski@samsung.com> [161013 23:37]:
>>> On 10/13/2016 04:20 PM, Matt Ranostay wrote:
>>>> On Thu, Oct 13, 2016 at 4:05 PM, Jacek Anaszewski
>>>> <j.anaszewski@samsung.com> wrote:
>>>>> Why DT property? Is it somehow dependent on the board configuration?
>>>>> How this period-scale value is calculated? Is it inferred empirically?
>>>>>
>>>>
>>>> We empirically discovered and verified this with an logic analyzer on
>>>> multiple batches of this part.
>>>> Reason for the DT entry is we aren't 100% sure that it is always going
>>>> to be the same with different board revs.
>>>>
>>>> Could be that parts clock acts differently with supply voltage.   This
>>>> has been calculated by setting it an expected value, and measuring the
>>>> actual result with the logic analyzer.
>>>
>>> I'd like to have DT maintainer's ack for this.
>>>
>>> Cc Rob and Mark.
>>
>> How about do this based on the compatible property instead? If there
>> are multiple manufacturers for this part and only a certain
>> parts have this issue we should have multiple compatible properties.
>>
>
> I could only find that NXP as the manufacturer of that part. It is
> possible since the clock is internal to the chipset that the vdd of
> 2.5V is doing something undefined.
>
>> Then if it turns out all of them need this scaling there's no need
>> to update the binding.
>
> Understandable.

Since at present we can't guarantee that all produced devices
are affected, then we should strive to avoid breaking any existing
users of the possible non-affected devices.

In view of that the addition of a new "compatible" proposed by Tony
seems most reasonable.

Still, DT maintainer's opinion is required.

-- 
Best regards,
Jacek Anaszewski