linux-kernel - Re: common non-cache coherent direct dma mapping ops

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Date:   Sun, 13 May 2018 15:26:20 +0200
From:   Helge Deller <deller@....de>
To:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     linux-arch@...r.kernel.org, Michal Simek <monstr@...str.eu>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        linux-alpha@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-c6x-dev@...ux-c6x.org,
        linux-hexagon@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        nios2-dev@...ts.rocketboards.org, openrisc@...ts.librecores.org,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: common non-cache coherent direct dma mapping ops

On 11.05.2018 09:59, Christoph Hellwig wrote:
> this series continues consolidating the dma-mapping code, with a focus
> on architectures that do not (always) provide cache coherence for DMA.
> Three architectures (arm, mips and powerpc) are still left to be
> converted later due to complexity of their dma ops selection.
> 
> The dma-noncoherent ops calls the dma-direct ops for the actual
> translation of streaming mappins and allow the architecture to provide
> any cache flushing required for cpu to device and/or device to cpu
> ownership transfers.  The dma coherent allocator is for now still left
> entirely to architecture supplied implementations due the amount of
> variations.  Hopefully we can do some consolidation for them later on
> as well.
> 
> A lot of architectures are currently doing very questionable things
> in their dma mapping routines, which are documented in the changelogs
> for each patch.  Please review them very careful and correct me on
> incorrect assumptions.
> 
> Because this series sits on top of two previously submitted series
> a git tree might be useful to actually test it.  It is provided here:
> 
>     git://git.infradead.org/users/hch/misc.git generic-dma-noncoherent
> 
> Gitweb:
> 
>     http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/generic-dma-noncoherent
> 
> Changes since RFC:
>  - fix a typo accidentally disabling the device to cpu transfer sync
>  - fixed a few compile failures


I tested it again on parisc (this time again on top of git head) and it still breaks
the same way as I reported in my mail on April 21st: the lasi82956 network driver works
unreliable. NIC gets IP, but ping doesn't work.
See drivers/net/ethernet/i825xx/lasi_82596.c, it uses dma*sync() functions.

See comment in James mail from April 21st too:
-> you just made every 32 bit parisc system unnecessarily use non-coherent.

Helge