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

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Message-ID: <CAGXJtFFjVTqYraM0OFipYnNbdLt_BojP5U8WhJETLgw4XWehGQ@mail.gmail.com>
Date:   Mon, 20 Aug 2018 13:39:06 -0700
From:   Alex Feinman <alex@...xfeinman.com>
To:     linux-kernel@...r.kernel.org
Subject: Contiguous DMA buffer view for a custom device (Intel/x86)

Good morning.
I have a custom PCI device that needs to fill memory buffers of up to 16MB
in size. For various reasons I would like to avoid implementing
scatter-gather support in the device and would much rather prefer a
contiguous buffer view instead. The buffers are allocated using
dma_alloc_coherent or dma_alloc_attrs (as I don't necessarily need coherent
access)

This is on a custom Skylake/Kaby Lake platform

It is my understanding (possibly incorrect) that
a) The contiguous memory space available to subsystems like
videobuf2-dma-contig is limited and I won't be likely to be able tp
allocate say 4 16MB contiguous buffers reliably
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)

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?

Are there any pointers of how exactly achieve this (implementation I could
peruse, articles and whatnot)?

Best regards

Alex Feinman
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