linux-kernel - Re: [PATCH 4/4] usb: chipidea: USB_CHIPIDEA should depend on HAS

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Date:	Tue, 27 Aug 2013 21:26:54 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Felipe Balbi <balbi@...com>
Cc:	Peter Chen <peter.chen@...escale.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	USB list <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dan Williams <djbw@...com>, Arnd Bergmann <arnd@...db.de>,
	Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 4/4] usb: chipidea: USB_CHIPIDEA should depend on HAS_DMA

On Tue, Aug 27, 2013 at 9:16 PM, Felipe Balbi <balbi@...com> wrote:
> On Mon, Aug 19, 2013 at 09:24:01AM +0800, Peter Chen wrote:
>> On Sun, Aug 18, 2013 at 10:20:44PM +0200, Geert Uytterhoeven wrote:
>> > If NO_DMA=y:
>> >
>> > drivers/built-in.o: In function `dma_set_coherent_mask':
>> > include/linux/dma-mapping.h:93: undefined reference to `dma_supported'
>> >
>> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> > ---
>> >  drivers/usb/chipidea/Kconfig |    2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/usb/chipidea/Kconfig b/drivers/usb/chipidea/Kconfig
>> > index d1bd8ef..dbd5232 100644
>> > --- a/drivers/usb/chipidea/Kconfig
>> > +++ b/drivers/usb/chipidea/Kconfig
>> > @@ -1,6 +1,6 @@
>> >  config USB_CHIPIDEA
>> >     tristate "ChipIdea Highspeed Dual Role Controller"
>> > -   depends on USB || USB_GADGET
>> > +   depends on (USB || USB_GADGET) && HAS_DMA
>> >     help
>> >       Say Y here if your system has a dual role high speed USB
>> >       controller based on ChipIdea silicon IP. Currently, only the
>>
>> I can't understand why the DMA can't be changed to fix this instead
>> of changing every driver?
>
> I'm with you. It'd be nicer to just provide empty stubs when !HAS_DMA

That would give us drivers that compile but don't work.
Not having stubs is intentional, cfr. the comment at the top of
include/asm-generic/dma-mapping-broken.h:

/* define the dma api to allow compilation but not linking of
 * dma dependent code.  Code that depends on the dma-mapping
 * API needs to set 'depends on HAS_DMA' in its Kconfig
 */

and the comments in the commit that introduced it:

commit 1b0fac45878bb88759eec347c273285195649ff7
Author: Dan Williams <dan.j.williams@...el.com>
Date:   Sun Jul 15 23:40:26 2007 -0700

    dma-mapping: prevent dma dependent code from linking on !HAS_DMA archs

    Continuing the work started in 411f0f3edc141a582190d3605cadd1d993abb6df ...

    This enables code with a dma path, that compiles away, to build without
    requiring additional code factoring.  It also prevents code that calls
    dma_alloc_coherent and dma_free_coherent from linking whereas previously
    the code would hit a BUG() at run time.  Finally, it allows archs that set
    !HAS_DMA to delete their asm/dma-mapping.h file.

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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