Messages in this thread | | | Date | Mon, 11 Apr 2022 18:39:37 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH 02/10] drivers/base: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN |
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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@arm.com> 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()
From the above, we can conclude that the data[] pointer is at a multiple of arch_kmalloc_minalign():
devres.data = (p + m * n) * arch_kmalloc_minalign()
Where m, n, p are all positive integers (n is also power of two).
If we did not change the devres structure, the alignment of ARCH_KMALLOC_MINALIGN would no no be longer sufficient since the dynamic arch_kmalloc_minalign() can be greater than ARCH_KMALLOC_MINALIGN on specific SoCs (the first offsetof equation is no longer true).
-- Catalin
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