Re: [PATCH 1/4] dma: define __dma_aligned attribute

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 21.09.23 11:56, Ahmad Fatoum wrote:
> Unlike the kernel, we always map barebox stack 1:1, so DMA to barebox
> stack is ok if care is taken for alignment. Otherwise, cache maintenance
> may end up clobbering data unintentionally.
> 
> Provide a __dma_aligned attribute for use in such situations and use the
> already existing DMA_ALIGNMENT as alignment.
> 
> To be able to do that, we need to make sure that the default DMA_ALIGNMENT
> is only defined when architectures don't define their own dma_alloc. If
> they do, they are responsible to define their own DMA_ALIGNMENT as well.
> 
> The new attribute is intentionally not called __cacheline_aligned,
> because it differs functionally: We care about the cache line size of
> the outer cache, while in Linux __cacheline_aligned is for L1 cache.
> A __dma_aligned wouldn't make sense for Linux as it would be too easy to
> abuse (e.g. placing it on VMAP_STACK), but for barebox, we do this at
> many places and an attribute would increase readability and even safety.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

Please dismiss this series. __dma_aligned as attribute only makes sense
if it would not only affect alignment, but also would round up variable
size to multiple of cache line to ensure, we don't touch adjacent unrelated
variables. I'll have to revisit this.

> ---
>  include/dma.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/dma.h b/include/dma.h
> index 2a09b747d1e2..469c482e7a3a 100644
> --- a/include/dma.h
> +++ b/include/dma.h
> @@ -17,17 +17,19 @@
>  
>  #define DMA_ADDRESS_BROKEN	NULL
>  
> +#ifndef dma_alloc
>  #ifndef DMA_ALIGNMENT
>  #define DMA_ALIGNMENT	32
>  #endif
>  
> -#ifndef dma_alloc
>  static inline void *dma_alloc(size_t size)
>  {
>  	return xmemalign(DMA_ALIGNMENT, ALIGN(size, DMA_ALIGNMENT));
>  }
>  #endif
>  
> +#define __dma_aligned __attribute__((__aligned__((DMA_ALIGNMENT))))
> +
>  #ifndef dma_free
>  static inline void dma_free(void *mem)
>  {

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |