Re: [RFC PATCH] dma-coherent: use KiB in DMA allocation logs

[Ankit Soni]

On Wed, Jun 24, 2026 at 12:05:15AM +0000, Vova Sharaienko wrote:
> We are proposing to update DMA reserved memory pool allocation log
> messages to display sizes in KiB instead of MiB. Using MiB caused
> allocations less than 1 MiB to be logged as 0 MiB due to integer
> truncation. KiB provides better precision for smaller memory regions
> specified in the Device Tree.
> 
> This is currently marked as an RFC because we would like feedback on
> a KiB unit change.
> 
> Signed-off-by: Vova Sharaienko <sharaienko@xxxxxxxxxx>
> ---
>  kernel/dma/coherent.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/coherent.c b/kernel/dma/coherent.c
> index 480dd1766ece..3ede8da395ce 100644
> --- a/kernel/dma/coherent.c
> +++ b/kernel/dma/coherent.c
> @@ -69,8 +69,8 @@ static struct dma_coherent_mem *dma_init_coherent_memory(phys_addr_t phys_addr,
>  	kfree(dma_mem);
>  out_unmap_membase:
>  	memunmap(mem_base);
> -	pr_err("Reserved memory: failed to init DMA memory pool at %pa, size %zd MiB\n",
> -		&phys_addr, size / SZ_1M);
> +	pr_err("Reserved memory: failed to init DMA memory pool at %pa, size %lu KiB\n",
> +		&phys_addr, (unsigned long)(size / SZ_1K));

'size' here is a size_t, so the portable specifier is %zu, which also
lets you drop the cast.

>  	return ERR_PTR(-ENOMEM);
>  }
>  
> @@ -384,8 +384,8 @@ static int __init rmem_dma_setup(unsigned long node, struct reserved_mem *rmem)
>  	}
>  #endif
>  
> -	pr_info("Reserved memory: created DMA memory pool at %pa, size %ld MiB\n",
> -		&rmem->base, (unsigned long)rmem->size / SZ_1M);
> +	pr_info("Reserved memory: created DMA memory pool at %pa, size %lu KiB\n",
> +		&rmem->base, (unsigned long)(rmem->size / SZ_1K));

rmem->size is a phys_addr_t, which can be 64-bit on a 32-bit kernel.
Casting to unsigned long may truncate. It's an unlikely corner case,
but casting to the widest(%llu) type can avoid it.

-Ankit

>  	return 0;
>  }
>  
> -- 
> 2.55.0.rc0.799.gd6f94ed593-goog
>