Re: [PATCH 1/2] iio: at91-sama5d2: Fix incorrect cast to platform_device

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 19 Oct 2021 10:29:28 +0200
Lars-Peter Clausen <lars@metafoo.de> wrote:

> The at91-sama5d2 driver calls `to_platform_device()` on a struct device
> that is part of a IIO device. This is incorrect since
> `to_platform_device()` must only be called on a struct device that is part
> of a platform device.
> 
> The code still works by accident because non of the struct platform_device
> specific fields are accessed.
> 
> Refactor the code a bit so that it behaves identically, but does not use
> the incorrect cast. This avoids accidentally adding undefined behavior in
> the future by assuming the `struct platform_device` is actually valid.
> 
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

This makes me nervous for the reason you give below.
Looking for a response from Eugen or someone else with access to the device
before I apply this one.

Thanks,

Jonathan

> ---
> The code is equivalent to before, but I'm a bit confused how this works.
> We call dma_request_chan() on the IIO device's struct device. Which should
> not yield any results.
> 
> Eugen can you check if/why this works and see if a follow up patch using
> the right struct device (the platform_device's) to request the DMA channel
> makes sense?
> ---
>  drivers/iio/adc/at91-sama5d2_adc.c | 34 ++++++++++++++----------------
>  1 file changed, 16 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/iio/adc/at91-sama5d2_adc.c b/drivers/iio/adc/at91-sama5d2_adc.c
> index 4c922ef634f8..3841e7b6c81d 100644
> --- a/drivers/iio/adc/at91-sama5d2_adc.c
> +++ b/drivers/iio/adc/at91-sama5d2_adc.c
> @@ -1661,10 +1661,9 @@ static int at91_adc_write_raw(struct iio_dev *indio_dev,
>  	}
>  }
>  
> -static void at91_adc_dma_init(struct platform_device *pdev)
> +static void at91_adc_dma_init(struct at91_adc_state *st)
>  {
> -	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
> -	struct at91_adc_state *st = iio_priv(indio_dev);
> +	struct device *dev = &st->indio_dev->dev;
>  	struct dma_slave_config config = {0};
>  	/* we have 2 bytes for each channel */
>  	unsigned int sample_size = st->soc_info.platform->nr_channels * 2;
> @@ -1679,9 +1678,9 @@ static void at91_adc_dma_init(struct platform_device *pdev)
>  	if (st->dma_st.dma_chan)
>  		return;
>  
> -	st->dma_st.dma_chan = dma_request_chan(&pdev->dev, "rx");
> +	st->dma_st.dma_chan = dma_request_chan(dev, "rx");
>  	if (IS_ERR(st->dma_st.dma_chan))  {
> -		dev_info(&pdev->dev, "can't get DMA channel\n");
> +		dev_info(dev, "can't get DMA channel\n");
>  		st->dma_st.dma_chan = NULL;
>  		goto dma_exit;
>  	}
> @@ -1691,7 +1690,7 @@ static void at91_adc_dma_init(struct platform_device *pdev)
>  					       &st->dma_st.rx_dma_buf,
>  					       GFP_KERNEL);
>  	if (!st->dma_st.rx_buf) {
> -		dev_info(&pdev->dev, "can't allocate coherent DMA area\n");
> +		dev_info(dev, "can't allocate coherent DMA area\n");
>  		goto dma_chan_disable;
>  	}
>  
> @@ -1704,11 +1703,11 @@ static void at91_adc_dma_init(struct platform_device *pdev)
>  	config.dst_maxburst = 1;
>  
>  	if (dmaengine_slave_config(st->dma_st.dma_chan, &config)) {
> -		dev_info(&pdev->dev, "can't configure DMA slave\n");
> +		dev_info(dev, "can't configure DMA slave\n");
>  		goto dma_free_area;
>  	}
>  
> -	dev_info(&pdev->dev, "using %s for rx DMA transfers\n",
> +	dev_info(dev, "using %s for rx DMA transfers\n",
>  		 dma_chan_name(st->dma_st.dma_chan));
>  
>  	return;
> @@ -1720,13 +1719,12 @@ static void at91_adc_dma_init(struct platform_device *pdev)
>  	dma_release_channel(st->dma_st.dma_chan);
>  	st->dma_st.dma_chan = NULL;
>  dma_exit:
> -	dev_info(&pdev->dev, "continuing without DMA support\n");
> +	dev_info(dev, "continuing without DMA support\n");
>  }
>  
> -static void at91_adc_dma_disable(struct platform_device *pdev)
> +static void at91_adc_dma_disable(struct at91_adc_state *st)
>  {
> -	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
> -	struct at91_adc_state *st = iio_priv(indio_dev);
> +	struct device *dev = &st->indio_dev->dev;
>  	/* we have 2 bytes for each channel */
>  	unsigned int sample_size = st->soc_info.platform->nr_channels * 2;
>  	unsigned int pages = DIV_ROUND_UP(AT91_HWFIFO_MAX_SIZE *
> @@ -1744,7 +1742,7 @@ static void at91_adc_dma_disable(struct platform_device *pdev)
>  	dma_release_channel(st->dma_st.dma_chan);
>  	st->dma_st.dma_chan = NULL;
>  
> -	dev_info(&pdev->dev, "continuing without DMA support\n");
> +	dev_info(dev, "continuing without DMA support\n");
>  }
>  
>  static int at91_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
> @@ -1770,9 +1768,9 @@ static int at91_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
>  	 */
>  
>  	if (val == 1)
> -		at91_adc_dma_disable(to_platform_device(&indio_dev->dev));
> +		at91_adc_dma_disable(st);
>  	else if (val > 1)
> -		at91_adc_dma_init(to_platform_device(&indio_dev->dev));
> +		at91_adc_dma_init(st);
>  
>  	/*
>  	 * We can start the DMA only after setting the watermark and
> @@ -1780,7 +1778,7 @@ static int at91_adc_set_watermark(struct iio_dev *indio_dev, unsigned int val)
>  	 */
>  	ret = at91_adc_buffer_prepare(indio_dev);
>  	if (ret)
> -		at91_adc_dma_disable(to_platform_device(&indio_dev->dev));
> +		at91_adc_dma_disable(st);
>  
>  	return ret;
>  }
> @@ -2077,7 +2075,7 @@ static int at91_adc_probe(struct platform_device *pdev)
>  	return 0;
>  
>  dma_disable:
> -	at91_adc_dma_disable(pdev);
> +	at91_adc_dma_disable(st);
>  per_clk_disable_unprepare:
>  	clk_disable_unprepare(st->per_clk);
>  vref_disable:
> @@ -2094,7 +2092,7 @@ static int at91_adc_remove(struct platform_device *pdev)
>  
>  	iio_device_unregister(indio_dev);
>  
> -	at91_adc_dma_disable(pdev);
> +	at91_adc_dma_disable(st);
>  
>  	clk_disable_unprepare(st->per_clk);
>