Re: [PATCH v2 0/4] IIO: Alignment fixes part 4 - bounce buffers for the hard cases.

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 13 Jun 2021 16:10:35 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Hi All,

If anyone has time to take a look at this, particularly the first patch which
does the interesting stuff and patch 3 which I don't think has had any review
yet that would be great.

Thanks,

Jonathan

> 
> Thanks to Andy and Nuno for reviews.
> 
> Chances since V1/RFC:
> * Renamed the function to iio_push_to_buffer_with_ts_unaligned()
> * Fixed the various bugs people pointed out.
> * Used more standard realloc handling to be more 'obviously' correct.
> * Added some additional comments on the sizing of the copy to explain why
>   it is a conservative estimate and may copy more than strictly necessary.
> 
> A few things we discussed I didn't do (for now)...
> 
> I decided against adding explicit bounce buffer allocation calls for now,
> though I'm open to doing that in future if we find doing the somewhat hidden
> realloc to be a problem.
> 
> I haven't computed a more precise data_sz as I don't thing the benefits
> of a more precise copy or not passing the size, make it worth the slight
> reduction in complexity for the callers.  Again, open to revisiting this
> in future!
> 
> I tested it by hacking the dummy driver to shift it's data by one
> byte and call iio_push_to_buffers_with_ts_unaligned().
> 
> Strictly a hack. I definitely don't want to move this driver over to this
> new interface as it might encourage inappropriate use.
> 
> diff --git a/drivers/iio/dummy/iio_simple_dummy_buffer.c b/drivers/iio/dummy/iio_simple_dummy_buffer.c
> index 59aa60d4ca37..b47af7df8efc 100644
> --- a/drivers/iio/dummy/iio_simple_dummy_buffer.c
> +++ b/drivers/iio/dummy/iio_simple_dummy_buffer.c
> @@ -19,6 +19,7 @@
>  #include <linux/iio/buffer.h>
>  #include <linux/iio/trigger_consumer.h>
>  #include <linux/iio/triggered_buffer.h>
> +#include <asm/unaligned.h>
>  
>  #include "iio_simple_dummy.h"
>  
> @@ -78,12 +79,13 @@ static irqreturn_t iio_simple_dummy_trigger_h(int irq, void *p)
>                         j = find_next_bit(indio_dev->active_scan_mask,
>                                           indio_dev->masklength, j);
>                         /* random access read from the 'device' */
> -                       data[i] = fakedata[j];
> +//                     data[i] = fakedata[j];
> +                       put_unaligned_le16(fakedata[j], ((u8 *)(&data[i])) + 1);
>                         len += 2;
>                 }
>         }
>  
> -       iio_push_to_buffers_with_timestamp(indio_dev, data,
> +       iio_push_to_buffers_with_ts_unaligned(indio_dev, ((u8 *)(data)) + 1, indio_dev->scan_bytes - 8,
>                                            iio_get_time_ns(indio_dev));
> 
> 
> v1 description:
> 
> I finally got around to do a manual audit of all the calls to
> iio_push_to_buffers_with_timestamp() which has the somewhat odd requirements
> of:
> 1. 8 byte alignment of the provided buffer.
> 2. space for an 8 byte naturally aligned timestamp to be inserted at the
>    end.
> 
> Unfortunately there were rather a lot of these left, but time to bite the bullet
> and clean them up.
> 
> As discussed previous in
> https://lore.kernel.org/linux-iio/20200920112742.170751-1-jic23@kernel.org/
> it is not easy to fix the alignment issue without requiring a bounce buffer.
> This final part of the 4 sets of fixes is concerned with the cases where
> bounce buffers are the proposed solutions.
> 
> In these cases we have hardware that reads a prefix that we wish to
> drop. That makes it hard to directly read the data into the correct location.
> 
> Rather than implementing bouce buffers in each case, this set provides some
> magic in the core to handle them via a new function.
> iio_push_to_buffers_with_ts_na() - non aligned
> 
> Note this is totally untested as I don't have suitable hardware or emulation.
> I can fake something up in the dummy driver or via QEMU but I definitely want
> both eyes and testing on this series!
> 
> Jonathan Cameron (4):
>   iio: core: Introduce iio_push_to_buffers_with_ts_unaligned()
>   iio: adc: ti-adc108s102: Fix alignment of buffer pushed to iio
>     buffers.
>   iio: gyro: mpu3050: Fix alignment and size issues with buffers.
>   iio: imu: adis16400: Fix buffer alignment requirements.
> 
>  drivers/iio/adc/ti-adc108s102.c   | 11 ++++----
>  drivers/iio/gyro/mpu3050-core.c   | 24 ++++++++--------
>  drivers/iio/imu/adis16400.c       | 20 ++++++++++----
>  drivers/iio/industrialio-buffer.c | 46 +++++++++++++++++++++++++++++++
>  include/linux/iio/buffer.h        |  4 +++
>  include/linux/iio/iio-opaque.h    |  4 +++
>  6 files changed, 86 insertions(+), 23 deletions(-)
>