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

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Message-ID: <f02f9a98-532b-d457-bbb5-21d874b43e8c@ozlabs.ru>
Date:   Tue, 24 Mar 2020 15:55:33 +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 14:37, Alexey Kardashevskiy wrote:
> 
> 
> 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,

Ah no, does not help much, simple setting dma_ops_bypass will though.


But eventually, in this function:

static inline bool dma_map_direct(struct device *dev,
               const struct dma_map_ops *ops)
{
       if (likely(!ops))
               return true;
       if (!dev->dma_ops_bypass)
               return false;

       return min_not_zero(*dev->dma_mask, dev->bus_dma_limit) >=
                           dma_direct_get_required_mask(dev);
}


we rather want it to take a dma handle and a size, and add

if (dev->bus_dma_limit)
	return dev->bus_dma_limit > dma_handle + size;


where dma_handle=phys_to_dma(dev, phys) (I am not doing it here as unmap
needs the same test and it does not receive phys as a parameter).




-- 
Alexey