linux-kernel - Re: [PATCH v7 06/10] iio: adc: Support ROHM BD79124 ADC

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Message-ID: <fb0cee75-30aa-4175-b518-cbf9f3b7d46c@gmail.com>
Date: Mon, 17 Mar 2025 09:07:30 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>,
 Nuno Sa <nuno.sa@...log.com>, David Lechner <dlechner@...libre.com>,
 Javier Carrasco <javier.carrasco.cruz@...il.com>,
 Olivier Moysan <olivier.moysan@...s.st.com>,
 Guillaume Stols <gstols@...libre.com>,
 Dumitru Ceclan <mitrutzceclan@...il.com>,
 Trevor Gamblin <tgamblin@...libre.com>,
 Matteo Martelli <matteomartelli3@...il.com>,
 Alisa-Dariana Roman <alisadariana@...il.com>,
 João Paulo Gonçalves <joao.goncalves@...adex.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org
Subject: Re: [PATCH v7 06/10] iio: adc: Support ROHM BD79124 ADC

On 14/03/2025 16:37, Andy Shevchenko wrote:
> On Fri, Mar 14, 2025 at 11:22:37AM +0200, Matti Vaittinen wrote:
>> On 13/03/2025 15:19, Andy Shevchenko wrote:
>>> On Thu, Mar 13, 2025 at 09:19:03AM +0200, Matti Vaittinen wrote:
> 
> ...
> 
>>>> +	ret = regmap_read(data->map, BD79124_REG_EVENT_FLAG_HI, &i_hi);
>>>> +	if (ret)
>>>> +		return IRQ_NONE;
>>>> +
>>>> +	ret = regmap_read(data->map, BD79124_REG_EVENT_FLAG_LO, &i_lo);
>>>> +	if (ret)
>>>> +		return IRQ_NONE;
>>>
>>> Only I don't get why you can't use bulk read here.
>>> The registers seem to be sequential.
>>
>> After taking another look - there seems to be undocumented register (0x1b)
>> between the BD79124_REG_EVENT_FLAG_HI (0x1a) and the
>> BD79124_REG_EVENT_FLAG_LO (0x1c).
>>
>> I won't touch it unless there is a real verified performance problem.
> 
> ...
> 
>>> In the similar way bulk write.
>>
>> definitely not due to the 0x1b.
> 
> Okay, it seems I misinterpreted the values you have in regmap configuration,
> I was under the impression that regmap is 16-bit data, but it is about address.
> 
> So, we need to know why the heck HW has sparse registers

We don't know. And we really don't even need to know it. We can just use 
this device knowing there are some.

> for what is supposed
> to be sequential. This needs a good comment.

I think it is quite usual that devices contain undocumented registers. 
Not sure having a comment that this device also has some, adds much of 
value? I suppose I can add a comment that we can't use bulk_write 
because registers aren't in subsequent addresses - but having just 
removed bunch of unnecessary comments from the code (as requested), I'm 
not sure adding this one really improves situation... When one sees 
separate reads/writes for data spread to multiple registers, he is 
likely to assume addresses aren't subsequent.

Yours,
	-- Matti