Re: [PATCH v2 04/16] iio: adc: max1027: Avoid device managed allocators for DMA purposes

[Jonathan Cameron <jic23@kernel.org>]

On Thu,  2 Sep 2021 23:14:25 +0200
Miquel Raynal <miquel.raynal@bootlin.com> wrote:

> My overall understanding [1] is that devm_ allocations will require an
> extra 16 bytes at the beginning of the allocated buffer to store
> information about the managing device, this shifts a little bit the
> buffer which may then not be fully aligned on cache lines, disqualifying
> them for being suitable for DMA.
> 
> Let's switch to a regular kmalloc based allocator to ensure st->buffer
> is DMA-able, as required by the IIO subsystem.
> 
> [1] https://linux-arm-kernel.infradead.narkive.com/vyJqy0RQ/question-devm-kmalloc-for-dma
> 
> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
As per the other thread (I was late to the discussion) I don't believe this is
necessary because there is no chance of the data in the cacheline being touched
by the CPU whilst DMA is going on.

https://linux-arm-kernel.infradead.narkive.com/vyJqy0RQ/question-devm-kmalloc-for-dma#post5

Writes before dma transfer are safe.  It's writes during it that can cause problem so as
long as a driver isn't doing devm_ calls during such writes (and I really hope this isn't)
then we are safe.  The device will only overwrite the data with what was already there.

Jonathan

> ---
>  drivers/iio/adc/max1027.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iio/adc/max1027.c b/drivers/iio/adc/max1027.c
> index f27044324141..1167d50c1d67 100644
> --- a/drivers/iio/adc/max1027.c
> +++ b/drivers/iio/adc/max1027.c
> @@ -438,9 +438,7 @@ static int max1027_probe(struct spi_device *spi)
>  	indio_dev->num_channels = st->info->num_channels;
>  	indio_dev->available_scan_masks = st->info->available_scan_masks;
>  
> -	st->buffer = devm_kmalloc_array(&indio_dev->dev,
> -					indio_dev->num_channels, 2,
> -					GFP_KERNEL);
> +	st->buffer = kmalloc_array(indio_dev->num_channels, 2, GFP_KERNEL);
>  	if (!st->buffer)
>  		return -ENOMEM;
>  
> @@ -451,7 +449,7 @@ static int max1027_probe(struct spi_device *spi)
>  						      NULL);
>  		if (ret < 0) {
>  			dev_err(&indio_dev->dev, "Failed to setup buffer\n");
> -			return ret;
> +			goto free_buffer;
>  		}
>  
>  		st->trig = devm_iio_trigger_alloc(&spi->dev, "%s-trigger",
> @@ -460,7 +458,7 @@ static int max1027_probe(struct spi_device *spi)
>  			ret = -ENOMEM;
>  			dev_err(&indio_dev->dev,
>  				"Failed to allocate iio trigger\n");
> -			return ret;
> +			goto free_buffer;
>  		}
>  
>  		st->trig->ops = &max1027_trigger_ops;
> @@ -470,7 +468,7 @@ static int max1027_probe(struct spi_device *spi)
>  		if (ret < 0) {
>  			dev_err(&indio_dev->dev,
>  				"Failed to register iio trigger\n");
> -			return ret;
> +			goto free_buffer;
>  		}
>  
>  		ret = devm_request_threaded_irq(&spi->dev, spi->irq,
> @@ -481,7 +479,7 @@ static int max1027_probe(struct spi_device *spi)
>  						st->trig);
>  		if (ret < 0) {
>  			dev_err(&indio_dev->dev, "Failed to allocate IRQ.\n");
> -			return ret;
> +			goto free_buffer;
>  		}
>  	}
>  
> @@ -490,7 +488,7 @@ static int max1027_probe(struct spi_device *spi)
>  	ret = spi_write(st->spi, &st->reg, 1);
>  	if (ret < 0) {
>  		dev_err(&indio_dev->dev, "Failed to reset the ADC\n");
> -		return ret;
> +		goto free_buffer;
>  	}
>  
>  	/* Disable averaging */
> @@ -498,10 +496,15 @@ static int max1027_probe(struct spi_device *spi)
>  	ret = spi_write(st->spi, &st->reg, 1);
>  	if (ret < 0) {
>  		dev_err(&indio_dev->dev, "Failed to configure averaging register\n");
> -		return ret;
> +		goto free_buffer;
>  	}
>  
>  	return devm_iio_device_register(&spi->dev, indio_dev);
> +
> +free_buffer:
> +	kfree(st->buffer);
> +
> +	return ret;
>  }
>  
>  static struct spi_driver max1027_driver = {