linux-kernel - Re: [PATCH 2/2] hwmon: Aspeed AST2400/AST2500 ADC

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Date:   Tue, 7 Mar 2017 15:24:20 -0800
From:   Rick Altherr <raltherr@...gle.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Joel Stanley <joel@....id.au>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, jdelvare@...e.com,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] hwmon: Aspeed AST2400/AST2500 ADC

On Sun, Mar 5, 2017 at 8:28 AM, Guenter Roeck <linux@...ck-us.net> wrote:
> On 03/01/2017 07:29 PM, Rick Altherr wrote:
>>
>> Resending in plain text.
>>
>> On Tue, Feb 28, 2017 at 7:45 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>> On 02/28/2017 04:49 PM, Joel Stanley wrote:
>>>>
>>>>
>>>> On Wed, Mar 1, 2017 at 6:44 AM, Rick Altherr <raltherr@...gle.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> Aspeed AST2400/AST2500 BMC SoCs include a 16 channel, 10-bit ADC. This
>>>>> driver implements reading the ADC values, enabling channels via device
>>>>> tree, and optionally providing channel labels via device tree.  Low and
>>>>> high threshold interrupts are supported by the hardware but not
>>>>> implemented.
>>>>>
>>>>> Signed-off-by: Rick Altherr <raltherr@...gle.com>
>>>>
>>>>
>>>>
>>>> Looks good. Some minor comments below.
>>>>
>>>> Is there a reason you wrote a hwmon driver instead of an iio driver? I
>>>> wasn't sure what the recommended subsystem is.
>>>
>>>
>>>
>>> Excellent point. Question is really if there is a plan to add support for
>>> thresholds. If not, an iio driver might be more appropriate.
>>>
>>> Guenter
>>>
>>
>> The hardware supports firing interrupts on high and low thresholds.
>> I'm not planning to use that feature yet so I didn't implement it.
>> Would you prefer that I leave it as is (maybe with a TODO), implement
>> the thresholding, or change to iio?
>>
>
> Let's try to determine the intended use of the ADC. I don't find the
> datasheet
> online; all I can find is brief summaries which don't me tell much, but
> suggest
> that it is a general purpose ADC and not specifically intended for hardware
> monitoring. What is the minimum sampling rate ? That should give us an idea.
> If it is in the uS range, iio would be more appropriate (and the iio-hwmon
> bridge could be used if a channel is in fact used for hardware monitoring).
>
> Thanks,
> Guenter
>

AST2500 is a BMC SoC from Aspeed
(https://www.aspeedtech.com/products.php?fPath=20&rId=440).  The BMC
is a separate, always-on computer that manages the health and remote
management for a server.  The driver I sent is for the ADCs included
in the SoC that are intended to monitor power rails on the server
motherboard but are really just general-purpose ADCs.  According to
the (not public) datasheet, the sampling rate is 10-500kHz, resolution
is 10-bit, and precision +/- 2%.  This is my first time writing a
linux driver for ADCs.  My cursory look at iio indicates that that
will work fine for this driver and the hwmon-iio bridge will suffice
for the userspace stack which is currently expecting hwmon APIs.