On Thu, 2 Sep 2021 23:14:25 +0200 Miquel Raynal <miquel.raynal@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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 = {
