linux-kernel - Re: [PATCH v3 0/2] swiotlb: allocate padding slots if necessary

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Message-ID: <20240322150941.GA5634@willie-the-truck>
Date: Fri, 22 Mar 2024 15:09:41 +0000
From: Will Deacon <will@...nel.org>
To: Petr Tesarik <petrtesarik@...weicloud.com>
Cc: Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Michael Kelley <mhklinux@...look.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
	Roberto Sassu <roberto.sassu@...weicloud.com>,
	Petr Tesarik <petr@...arici.cz>
Subject: Re: [PATCH v3 0/2] swiotlb: allocate padding slots if necessary

Hi Petr,

On Thu, Mar 21, 2024 at 06:19:00PM +0100, Petr Tesarik wrote:
> From: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> 
> If the allocation alignment is bigger than IO_TLB_SIZE and min_align_mask
> covers some bits in the original address between IO_TLB_SIZE and
> alloc_align_mask, preserve these bits by allocating additional padding
> slots before the actual swiotlb buffer.

Thanks for fixing this! I was out at a conference last week, so I didn't
get very far with it myself, but I ended up in a pickle trying to avoid
extending 'struct io_tlb_slot'. Your solution is much better than the
crazy avenue I started going down...

With your changes, can we now simplify swiotlb_align_offset() to ignore
dma_get_min_align_mask() altogether and just:

	return addr & (IO_TLB_SIZE - 1);

?

Will