linux-kernel - Re: Modifying sg_dma

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Message-ID: <20150519040147.GG3140@localhost>
Date:	Tue, 19 May 2015 09:31:47 +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 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...

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...

-- 
~Vinod
> 
> Is it allowed to modify sg_dma_len(sg)?
> Is it OK to restore the original sg_dma_len()?
> 
> Thanks for your answers!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
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