On Mon, Apr 11, 2022 at 05:57:08PM +0300, Andy Shevchenko wrote:
> On Wed, Apr 6, 2022 at 2:30 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > ARCH_DMA_MINALIGN represents the minimum (static) alignment for safe DMA
> > operations while ARCH_KMALLOC_MINALIGN is the minimum kmalloc() objects
> > alignment.
>
> ...
>
> > - * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same
> > + * Thus we use ARCH_DMA_MINALIGN here and get at least the same
> > * buffer alignment as if it was allocated by plain kmalloc().
>
> But then it becomes not true either, because the kmalloc() has other
> alignment constraints.
Maybe the comment could be improved a bit but I think it's still valid.
After this patch, struct devres becomes:
struct devres {
struct devres_node node;
u8 __aligned(ARCH_DMA_MINALIGN) data[];
};
While we no longer guarantee the ARCH_DMA_MINALIGN alignment (which is
too big on most arm64 SoCs), what we need is for devres.data[] to be
aligned to the newly introduced arch_kmalloc_minalign(). This would give
us the DMA safety guarantees.
Since devres.data[] is at an offset multiple of ARCH_DMA_MINALIGN, in
order for the array to be aligned to arch_kmalloc_minalign(), all we
need is for ARCH_DMA_MINALIGN to be a multiple of
arch_kmalloc_minalign(). I actually had to write down some simple
equations to convince myself.
devres.data[] is at an offset multiple of ARCH_DMA_MINALIGN (after this
patch), even when struct devres is included in another structure, so we
have:
offsetof(struct devres, data) = m * ARCH_DMA_MINALIGN
ARCH_DMA_MINALIGN is a power of two while arch_kmalloc_minalign() is
also a power of two, equal to or less than ARCH_DMA_MINALIGN:
ARCH_DMA_MINALIGN = n * arch_kmalloc_minalign()
A kmalloc()'ed object of struct devres (or a container of) is aligned to
arch_kmalloc_minalign() by definition so:
kmalloc() = p * arch_kmalloc_minalign()
