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

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