From: Will Deacon <will@kernel.org>
To: "Petr Tesařík" <petr@tesarici.cz>
Cc: Christoph Hellwig <hch@lst.de>,
Marek Szyprowski <m.szyprowski@samsung.com>,
Robin Murphy <robin.murphy@arm.com>,
Petr Tesarik <petr.tesarik1@huawei-partners.com>,
Michael Kelley <mhklinux@outlook.com>,
open list <linux-kernel@vger.kernel.org>,
"open list:DMA MAPPING HELPERS" <iommu@lists.linux.dev>,
Roberto Sassu <roberto.sassu@huaweicloud.com>
Subject: Re: [PATCH v3 0/2] swiotlb: allocate padding slots if necessary
Date: Wed, 27 Mar 2024 19:42:01 +0000 [thread overview]
Message-ID: <20240327194200.GA12000@willie-the-truck> (raw)
In-Reply-To: <20240322185138.1af83c61@meshulam.tesarici.cz>
On Fri, Mar 22, 2024 at 06:51:38PM +0100, Petr Tesařík wrote:
> On Fri, 22 Mar 2024 15:09:41 +0000
> Will Deacon <will@kernel.org> wrote:
>
> > Hi Petr,
> >
> > On Thu, Mar 21, 2024 at 06:19:00PM +0100, Petr Tesarik wrote:
> > > From: Petr Tesarik <petr.tesarik1@huawei-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);
>
> I have also thought about this but I don't think it's right. If we
> removed dma_get_min_align_mask() from swiotlb_align_offset(), we would
> always ask to preserve the lowest IO_TLB_SHIFT bits. This may cause
> less efficient use of the SWIOTLB.
>
> For example, if a device does not specify any min_align_mask, it is
> presumably happy with any buffer alignment, so SWIOTLB may allocate at
> the beginning of a slot, like here:
>
> orig_addr | ++|++ |
> tlb_addr |++++ | |
>
> Without dma_get_min_align_mask() in swiotlb_align_offset(), it would
> have to allocate two mostly-empty slots:
>
> tlb_addr | ++|++ |
>
> where:
> | mark a multiple of IO_TLB_SIZE (in physical address space)
> + used memory
> free memory
Thanks for the patient explanation. I'd got so caught up with the DMA
alignment mask that I forgot the usual case where it's not specified at
all!
Will
next prev parent reply other threads:[~2024-03-27 19:42 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-21 17:19 [PATCH v3 0/2] swiotlb: allocate padding slots if necessary Petr Tesarik
2024-03-21 17:19 ` [PATCH v3 1/2] swiotlb: extend buffer pre-padding to alloc_align_mask " Petr Tesarik
2024-03-22 4:29 ` Michael Kelley
2024-03-22 10:29 ` Petr Tesarik
2024-03-21 17:19 ` [PATCH v3 2/2] bug: introduce ASSERT_VAR_CAN_HOLD() Petr Tesarik
2024-03-22 4:33 ` Michael Kelley
2024-03-22 15:37 ` Will Deacon
2024-03-22 17:33 ` Petr Tesařík
2024-03-22 15:09 ` [PATCH v3 0/2] swiotlb: allocate padding slots if necessary Will Deacon
2024-03-22 17:51 ` Petr Tesařík
2024-03-27 19:42 ` Will Deacon [this message]
2024-03-22 18:11 ` Michael Kelley
2024-03-22 19:31 ` Petr Tesařík
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