linux-kernel - Re: UBIFS with dma on 4.6 kernel is not working

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Date:   Fri, 21 Oct 2016 15:07:57 +0200
From:   Richard Weinberger <richard@....at>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Naga Sureshkumar Relli <naga.sureshkumar.relli@...inx.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "computersforpeace@...il.com" <computersforpeace@...il.com>,
        "dedekind1@...il.com" <dedekind1@...il.com>,
        "adrian.hunter@...el.com" <adrian.hunter@...el.com>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        Punnaiah Choudary Kalluri <punnaia@...inx.com>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>
Subject: Re: UBIFS with dma on 4.6 kernel is not working

Christoph,

On 21.10.2016 14:53, Christoph Hellwig wrote:
> On Fri, Oct 21, 2016 at 11:29:16AM +0200, Richard Weinberger wrote:
>> DMA to vmalloced memory not good, it may work by chance if you transfer
>> less than PAGE_SIZE.
>> Especially on ARM.
> 
> DMA to vmalloc'ed or vmap memory is perfectly fine, you just have to be
> very careful.
> 
> I would suggest to not expose the vmalloc address to the lower layers
> that do DMA, but instead expose the pages, either as an array or
> scatterlist.  Either allocate the pages using the normal page allocator
> and then use vm_map_ram to generate a virtual address for them (that
> is what XFS does for it's large metadata objects for example).  Or if
> you can't do that iterate over the vmalloc address in page size chunks
> and use vmalloc_to_page (we still also do that for one piece of legacy
> cruft in XFS, but I'd rather avoid that for new designs).
> 

Hmm, thought this is still problematic on VIVT architectures.
Boris tried to provide a solution for that some time ago:
http://www.spinics.net/lists/arm-kernel/msg494025.html

Thanks,
//richard