From: Christoph Hellwig <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Nicolin Chen <nicoleotsuka@gmail.com>,
linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
hch@lst.de
Subject: Re: [RFC][PATCH] dma-mapping: align default segment_boundary_mask with dma_mask
Date: Mon, 16 Mar 2020 13:46:52 +0100 [thread overview]
Message-ID: <20200316124652.GA17386@lst.de> (raw)
In-Reply-To: <f36ac67e-0eca-46df-78ec-c8b1c4fbe951@arm.com>
On Mon, Mar 16, 2020 at 12:12:08PM +0000, Robin Murphy wrote:
> On 2020-03-14 12:00 am, Nicolin Chen wrote:
>> More and more drivers set dma_masks above DMA_BIT_MAKS(32) while
>> only a handful of drivers call dma_set_seg_boundary(). This means
>> that most drivers have a 4GB segmention boundary because DMA API
>> returns DMA_BIT_MAKS(32) as a default value, though they might be
>> able to handle things above 32-bit.
>
> Don't assume the boundary mask and the DMA mask are related. There do exist
> devices which can DMA to a 64-bit address space in general, but due to
> descriptor formats/hardware design/whatever still require any single
> transfer not to cross some smaller boundary. XHCI is 64-bit yet requires
> most things not to cross a 64KB boundary. EHCI's 64-bit mode is an example
> of the 4GB boundary (not the best example, admittedly, but it undeniably
> exists).
Yes, which is what the boundary is for. But why would we default to
something restrictive by default even if the driver didn't ask for it?
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next prev parent reply other threads:[~2020-03-16 12:47 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-14 0:00 [RFC][PATCH] dma-mapping: align default segment_boundary_mask with dma_mask Nicolin Chen
2020-03-16 10:45 ` Christoph Hellwig
2020-03-16 12:12 ` Robin Murphy
2020-03-16 12:46 ` Christoph Hellwig [this message]
2020-03-16 13:16 ` Robin Murphy
2020-03-16 21:42 ` Nicolin Chen
2020-03-16 21:39 ` Nicolin Chen
2020-03-16 12:48 ` Christoph Hellwig
2020-03-16 21:45 ` Nicolin Chen
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