linux-kernel - Re: Contiguous DMA buffer view for a custom device (Intel/x86)

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Date:   Mon, 20 Aug 2018 21:58:04 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Alex Feinman <alex@...xfeinman.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Contiguous DMA buffer view for a custom device (Intel/x86)

> b) IOMMU can solve this problem for me by providing a device-specific
> contiguous view of a fragmented physical memory allocation
> c) In order to enable IOMMU do the above, I need to allocate DRHDs and
> DMARs in BIOS initialization (I build my own BIOS)

Yes. The EDK2 firmware toolkit has all the bits you need in it I think.

https://firmware.intel.com/sites/default/files/Intel_WhitePaper_Using_IOMMU_for_DMA_Protection_in_UEFI.pdf

isn't quite on the topic you want but it does explain it fairly well in
passing.

https://software.intel.com/en-us/blogs/2009/03/02/intels-virtualization-for-directed-io-aka-iommu-part-1

is a bit out of date but may help too.

> Please, let me know if I am on the right track? Of course I realize that
> implementing SGDMA would be the best option, but short of that and blocking
> out some physical memory on boot, what are my options?

If performance is absolutely critical simply stealing a chunk of memory
in the firmware and describing it your device some other way is ugly, but
for a custom solution I guess anything goes 8)

Alan