linux-kernel - Re: [PATCH 1/2] dma-mapping: add a dma_ops

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Date:   Tue, 24 Mar 2020 14:37:59 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device



On 24/03/2020 04:20, Christoph Hellwig wrote:
> On Mon, Mar 23, 2020 at 07:58:01PM +1100, Alexey Kardashevskiy wrote:
>>>> 0x100.0000.0000 .. 0x101.0000.0000
>>>>
>>>> 2x4G, each is 1TB aligned. And we can map directly only the first 4GB
>>>> (because of the maximum IOMMU table size) but not the other. And 1:1 on
>>>> that "pseries" is done with offset=0x0800.0000.0000.0000.
>>>>
>>>> So we want to check every bus address against dev->bus_dma_limit, not
>>>> dev->coherent_dma_mask. In the example above I'd set bus_dma_limit to
>>>> 0x0800.0001.0000.0000 and 1:1 mapping for the second 4GB would not be
>>>> tried. Does this sound reasonable? Thanks,
>>>
>>> bus_dma_limit is just another limiting factor applied on top of
>>> coherent_dma_mask or dma_mask respectively.
>>
>> This is not enough for the task: in my example, I'd set bus limit to
>> 0x0800.0001.0000.0000 but this would disable bypass for all RAM
>> addresses - the first and the second 4GB blocks.
> 
> So what about something like the version here:
> 
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-bypass.3


dma_alloc_direct() and dma_map_direct() do the same thing now which is
good, did I miss anything else?

This lets us disable bypass automatically if this weird memory appears
in the system but does not let us have 1:1 after that even for normal
RAM. Thanks,


-- 
Alexey