linux-kernel - Re: dma_mmap

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Message-ID: <20190906063203.GA25415@infradead.org>
Date:   Thu, 5 Sep 2019 23:32:03 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 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.