linux-kernel - Re: [PATCH 2/2] swiotlb: Add swiotlb=nobounce debug option

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Date:   Mon, 7 Nov 2016 19:57:11 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Jonathan Corbet <corbet@....net>,
        Magnus Damm <magnus.damm@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        iommu@...ts.linux-foundation.org,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 2/2] swiotlb: Add swiotlb=nobounce debug option

Hi Konrad,

On Mon, Oct 31, 2016 at 6:52 PM, Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
> On Mon, Oct 31, 2016 at 04:45:04PM +0100, Geert Uytterhoeven wrote:
>> On architectures like arm64, swiotlb is tied intimately to the core
>> architecture DMA support. In addition, ZONE_DMA cannot be disabled.
>>
>> To aid debugging and catch devices not supporting DMA to memory outside
>> the 32-bit address space, add a kernel command line option
>> "swiotlb=nobounce", which disables the use of bounce buffers.
>> If specified, trying to map memory that cannot be used with DMA will
>> fail, and a warning will be printed (rate-limited).
>
> I would make the 'swiotlb_force' an enum. And then instead of this
> being 'nobounce' just do the inverse of 'force', that is the
> 'noforce' would trigger this no bounce effect.
>
> So:
>
> enum {
>         NORMAL,         /* Default - depending on the hardware DMA mask and such. */
>         FORCE,          /* swiotlb=force */
>         NO_FORCE,       /* swiotlb=noforce */

Fine for me, but swiotlb_force is exported to platform code. Hence all users
should be updated?

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