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

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Message-ID: <CAGXJtFG+9N_X_pn1XW02aqxuMKU_icaY55m9q3expxDvdv2TgQ@mail.gmail.com>
Date:   Tue, 21 Aug 2018 16:50:22 -0700
From:   Alex Feinman <alex@...xfeinman.com>
To:     gnomes@...rguk.ukuu.org.uk
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Contiguous DMA buffer view for a custom device (Intel/x86)

Thanks, this sounds like what I need. However, since this is a
custom-purpose system, I am tempted to go with CMA and simply
preallocated 200MB on start

On Mon, Aug 20, 2018 at 1:58 PM Alan Cox <gnomes@...rguk.ukuu.org.uk> wrote:
>
> > 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 is a bit out of date but may help too.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