linux-kernel - Re: [PATCH v2] mtd: onenand: omap2: Disable DMA for HIGHMEM buffers

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Date:   Fri, 20 Apr 2018 22:01:34 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Ladislav Michl <ladis@...ux-mips.org>
Cc:     linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Roger Quadros <rogerq@...com>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        Tony Lindgren <tony@...mide.com>,
        "H. Nikolaus Schaller" <hns@...delico.com>,
        Andreas Kemnade <andreas@...nade.info>
Subject: Re: [PATCH v2] mtd: onenand: omap2: Disable DMA for HIGHMEM buffers

Hi Ladislav,

On Mon, 16 Apr 2018 08:52:59 +0200
Ladislav Michl <ladis@...ux-mips.org> wrote:

> dma_map_single doesn't get the proper DMA address for vmalloced area,

That's not true, it returns the right DMA (physical) address, it's just
that:

1/ the memory location is not necessarily physically contiguous
2/ in case your arch is VIVT ot VIPT, there may be several entries in
   the cache pointing to the same physical location, and the cache
   maintenance operations done by dma_map_single() will only operate on
   one of these entries.

> so disable DMA in this case.
> 

The fix looks good though. Can you rephrase your commit message to make
it clearer.

Thanks,

Boris