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

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Message-ID: <ZJoAZgn2b7RcofzY@nixie71>
Date:   Mon, 26 Jun 2023 16:17:26 -0500
From:   Jeff LaBundy <jeff@...undy.com>
To:     Neil Armstrong <neil.armstrong@...aro.org>
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 Neil,

On Mon, Jun 26, 2023 at 03:20:35PM +0200, Neil Armstrong wrote:
> 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.

I revisted the regmap code, and you are indeed correct. I agree with your
suggestion.

> 
> Neil
> 
> > 
> > Kind regards,
> > Jeff LaBundy
> 

Kind regards,
Jeff LaBundy