linux-kernel - Re: [PATCH v3 4/4] input: touchscreen: add SPI support for Goodix Berlin Touchscreen IC

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Date:   Mon, 26 Jun 2023 15:20:35 +0200
From:   Neil Armstrong <neil.armstrong@...aro.org>
To:     Jeff LaBundy <jeff@...undy.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bastien Nocera <hadess@...ess.net>,
        Hans de Goede <hdegoede@...hat.com>,
        Henrik Rydberg <rydberg@...math.org>,
        linux-input@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] input: touchscreen: add SPI support for Goodix
 Berlin Touchscreen IC

Hi,

On 26/06/2023 15:01, Jeff LaBundy wrote:
> Hi Neil,
> 
> On Mon, Jun 26, 2023 at 09:02:16AM +0200, Neil Armstrong wrote:
> 
> [...]
> 
>>>> +static int goodix_berlin_spi_probe(struct spi_device *spi)
>>>> +{
>>>> +	struct regmap_config *regmap_config;
>>>> +	struct regmap *regmap;
>>>> +	size_t max_size;
>>>> +	int error = 0;
>>>> +
>>>> +	regmap_config = devm_kmemdup(&spi->dev, &goodix_berlin_spi_regmap_conf,
>>>> +				     sizeof(*regmap_config), GFP_KERNEL);
>>>> +	if (!regmap_config)
>>>> +		return -ENOMEM;
>>>
>>> Is there any reason we cannot simply pass goodix_berlin_spi_regmap_conf to
>>> devm_regmap_init() below? Why to duplicate and pass the copy?
>>>
>>> For reference, BMP280 in IIO is a similar example of a device with regmap
>>> sitting atop a bespoke SPI protocol; it does not seem to take this extra
>>> step.
>>
>> The goodix_berlin_spi_regmap_conf copy is modified after with the correct
>> max raw read/write size, and I'm not a fan of modifying a global structure
>> that could be use for multiple probes, I can make a copy in a stack variable
>> if it feels simpler.
> 
> Ah, that makes sense; in that case, the existing implementation seems fine
> to me. No changes necessary.
> 
> Correct me if I'm wrong, but the stack variable method wouldn't work since
> that memory is gone after goodix_berlin_spi_probe() returns.

The config is only needed for the devm_regmap_init() duration, so keeping
the memory allocated for the whole lifetime of the device seems useless.

Neil

> 
> Kind regards,
> Jeff LaBundy