linux-kernel - Re: [PATCH] MIPS: Fix `dma_alloc_coherent' returning a non-coherent allocation

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Date:   Thu, 1 Nov 2018 14:30:34 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@...ux-mips.org>
To:     Christoph Hellwig <hch@....de>
cc:     Paul Burton <paul.burton@...s.com>,
        iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] MIPS: Fix `dma_alloc_coherent' returning a non-coherent
 allocation

On Thu, 1 Nov 2018, Christoph Hellwig wrote:

> Oops, yes this looks good:

 BTW, for anyone missing hardware suitable for serious DMA testing I can 
recommend getting a pair of DEFPA cards, the ubiquitous PCI version of 
this board, cheaply available, which is the same except for a different 
host bus bridge ASIC, developed later.  They can be wired back to back 
similarly to Ethernet adapters (or in a loop if you have 2 or more dual 
attachment versions), no other hardware is required save for patch cords.
Version 3 (DEFPA-xC) boards support universal PCI signalling, older ones 
are 5V-only.

 These devices are a stellar example of fine engineering[1].  Our `defxx' 
driver, which I believe has been adapted from the DEC OSF/1 one referred 
in the said document, has some latency and other issues that I plan to 
address sometime, once I have sorted higher-priority issues, however 
hardware itself is excellent.

References:

[1] Chran-Ham Chang et al., "High-performance TCP/IP and UDP/IP Networking 
    in DEC OSF/1 for Alpha AXP", Digital Technical Journal, vol. 5, no. 1 
    (Winter 1993), "Network Adapter Characteristics", p. 7
    <ftp://ftp.linux-mips.org/pub/linux/mips/people/macro/DEC/DTJ/DTJ904/DTJ904PF.PDF>

  Maciej