Re: [PATCH 4.4 03/16] devres: Align data[] to ARCH_KMALLOC_MINALIGN

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Apr 23, 2020 at 09:40:01PM +0100, Lee Jones wrote:
> From: Alexey Brodkin <alexey.brodkin@synopsys.com>
> 
> [ Upstream commit a66d972465d15b1d89281258805eb8b47d66bd36 ]
> 
> Initially we bumped into problem with 32-bit aligned atomic64_t
> on ARC, see [1]. And then during quite lengthly discussion Peter Z.
> mentioned ARCH_KMALLOC_MINALIGN which IMHO makes perfect sense.
> If allocation is done by plain kmalloc() obtained buffer will be
> ARCH_KMALLOC_MINALIGN aligned and then why buffer obtained via
> devm_kmalloc() should have any other alignment?
> 
> This way we at least get the same behavior for both types of
> allocation.
> 
> [1] http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004009.html
> [2] http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004036.html
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: David Laight <David.Laight@ACULAB.COM>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vineet Gupta <vgupta@synopsys.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Greg KH <greg@kroah.com>
> Cc: <stable@vger.kernel.org> # 4.8+
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Lee Jones <lee.jones@linaro.org>
> ---
>  drivers/base/devres.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index 8fc654f0807bf..9763325a9c944 100644
> --- a/drivers/base/devres.c
> +++ b/drivers/base/devres.c
> @@ -24,8 +24,14 @@ struct devres_node {
>  
>  struct devres {
>  	struct devres_node		node;
> -	/* -- 3 pointers */
> -	unsigned long long		data[];	/* guarantee ull alignment */
> +	/*
> +	 * Some archs want to perform DMA into kmalloc caches
> +	 * and need a guaranteed alignment larger than
> +	 * the alignment of a 64-bit integer.
> +	 * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same
> +	 * buffer alignment as if it was allocated by plain kmalloc().
> +	 */
> +	u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
>  };
>  
>  struct devres_group {
> -- 
> 2.25.1
> 

I don't want to apply this to older kernels as it could cause extra
memory usage for no good reason.  I have no idea why a non ARC system
would want it :(

greg k-h