linux-kernel - Re: Modifying sg_dma

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Date:	Thu, 21 May 2015 21:23:46 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	dmaengine@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Modifying sg_dma_len(sg)?

On Tue, May 19, 2015 at 08:51:54AM +0200, Geert Uytterhoeven wrote:
> On Tue, May 19, 2015 at 6:01 AM, Vinod Koul <vinod.koul@...el.com> wrote:
> > On Fri, May 15, 2015 at 03:46:27PM +0200, Geert Uytterhoeven wrote:
> >
> > am ccing LKML, perhaps this needs wider discussion..
> >
> >> Several drivers reuse mapped scatterlists, and modify sg_dma_len(sg) to
> >> match the actual number of bytes they want to transfer.
> >>
> >> Hence during driver shutdown, dma_unmap_sg() is called with a scatterlist
> >> that has modified lengths, compared to the original passed to dma_map_sg().
> >> If CONFIG_DMA_API_DEBUG=y, this causes warnings like:
> >>
> >>     WARNING: CPU: 0 PID: 20 at lib/dma-debug.c:1103 check_unmap+0x24c/0x85c()
> >>     rcar-dmac e6700000.dma-controller: DMA-API: device driver frees
> >> DMA memory with different size [device address=0x000000006e15f000]
> >> [map size=4096 bytes] [unmap size=3 bytes]
> >>
> >> The warning can be silenced by restoring the original sg_dma_len() just before
> >> calling dma_unmap_sg(), but it looks like no driver actually does that.
> > Right, but should the drivers map with one length and then modify the
> > length later on, why not map only the size that is required...
> 
> This is mostly done in serial drivers, which set up an initial mapping, and
> reuse it with different lengths, depending on the amount of data to transfer
> at that time.
> 
> > So driver should always unmap and remap with the right length whenever it is
> > required to change the length. Perhaps a dma_remap_sg() API to do so
> > properly.
> >
> > I do not think modifying length directly should be encouraged...
> 
> In cases where there's only a single segment, I think it's better to use
> dma_map_single() instead of dma_map_sg().
> Then the actual length to transfer can easily be passed to
> dmaengine_prep_slave_single(), right?
Sounds right to me

-- 
~Vinod

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