linux-kernel - Re: [PATCH v5 1/7] iio: make IIO_DMA

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Message-ID: <20250507210239.138b3e0a@jic23-huawei>
Date: Wed, 7 May 2025 21:02:39 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: David Lechner <dlechner@...libre.com>
Cc: Nuno Sá <nuno.sa@...log.com>, Andy Shevchenko
 <andy@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, Michael Hennerich
 <Michael.Hennerich@...log.com>, Eugen Hristev <eugen.hristev@...aro.org>,
 Nicolas Ferre <nicolas.ferre@...rochip.com>, Alexandre Belloni
 <alexandre.belloni@...tlin.com>, Claudiu Beznea <claudiu.beznea@...on.dev>,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 1/7] iio: make IIO_DMA_MINALIGN minimum of 8 bytes

On Mon, 05 May 2025 11:31:42 -0500
David Lechner <dlechner@...libre.com> wrote:

> Add a condition to ensure that IIO_DMA_MINALIGN is at least 8 bytes.
> On some 32-bit architectures, IIO_DMA_MINALIGN is 4. In many cases,
> drivers are using this alignment for buffers that include a 64-bit
> timestamp that is used with iio_push_to_buffers_with_ts(), which expects
> the timestamp to be aligned to 8 bytes. To handle this, we can just make
> IIO_DMA_MINALIGN at least 8 bytes.
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
Dropped again...

In file included from drivers/input/touchscreen/tsc2007_iio.c:8:
./include/linux/iio/iio.h:784:5: warning: '__alignof__' is not defined, evaluates to 0 [-Wundef]
  784 | #if ARCH_DMA_MINALIGN < sizeof(s64)
      |     ^
./include/linux/cache.h:181:27: note: expanded from macro 'ARCH_DMA_MINALIGN'
  181 | #define ARCH_DMAmake[3]: *** [scripts/Makefile.build:461: drivers/hid] Error 2
_MINALIGN __alignof__(unsigned long long)
      |                           ^
In file included from drivers/input/touchscreen/tsc2007_iio.c:8:
./include/linux/iio/iio.h:784:5: error: function-like macro '__alignof__' is not defined
./include/linux/cache.h:181:27: note: expanded from macro 'ARCH_DMA_MINALIGN'
  181 | #define ARCH_DMA_MINALIGN __alignof__(unsigned long long)
      |   

With an LLVM build on x86.

> ---
>  include/linux/iio/iio.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index 638cf2420fbd85cf2924d09d061df601d1d4bb2a..7e1e3739328d103262071bd34ba5f6631163c122 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -775,8 +775,18 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
>   * to in turn include IIO_DMA_MINALIGN'd elements such as buffers which
>   * must not share  cachelines with the rest of the structure, thus making
>   * them safe for use with non-coherent DMA.
> + *
> + * A number of drivers also use this on buffers that include a 64-bit timestamp
> + * that is used with iio_push_to_buffer_with_ts(). Therefore, in the case where
> + * DMA alignment is not sufficient for proper timestamp alignment, we align to
> + * 8 bytes instead.
>   */
> +#if ARCH_DMA_MINALIGN < sizeof(s64)
> +#define IIO_DMA_MINALIGN sizeof(s64)
> +#else
>  #define IIO_DMA_MINALIGN ARCH_DMA_MINALIGN
> +#endif
> +
>  struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv);
>  
>  /* The information at the returned address is guaranteed to be cacheline aligned */
>