linux-kernel - Re: dma_mmap

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Date:   Fri, 6 Sep 2019 09:10:08 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        "pv-drivers@...are.com" <pv-drivers@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Christian König <christian.koenig@....com>
Subject: Re: dma_mmap_fault discussion

On 9/6/19 8:32 AM, Christoph Hellwig wrote:
> On Thu, Sep 05, 2019 at 07:05:43PM +0200, Thomas Hellström (VMware) wrote:
>> I took a quick look at the interfaces and have two questions:
>>
>> 1) dma_mmap_prepare(), would it be possible to drop the references to the
>> actual coherent region? The thing is that TTM doesn't know at mmap time what
>> memory will be backing the region. It can be VRAM, coherent memory or system
> I guess we can shift the argument checking into the fault handler.
>
>> 2) @cpu_addr and @dma_addr are the values pointing at the beginning of an
>> allocated chunk, right?
> Yes.
>
>> The reason I'm asking is that TTM's coherent memory
>> pool is sub-allocating from larger chunks into smaller PAGE_SIZE chunks,
>> which means that a TTM buffer object may be randomly split across larger
>> chunks, which means we have to store these values for each PAGE_SiZE chunk.
> For implementations that remap non-contigous regions using vmap we need the
> start cpu address passed, as that is used to find the vm_struct structure
> for it.  That being said I don't see a problem with you keeping track
> of the original start and offset into in your suballocation helpers.

It's definitely possible. I was just wondering whether it was necessary, 
but it seems like it.

Thanks,

Thomas