linux-kernel - Re: [PATCH] usb: USB host support should depend on HAS

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Date:	Thu, 11 Jul 2013 01:12:56 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	linux-input@...r.kernel.org, linux-media@...r.kernel.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: USB host support should depend on HAS_DMA

On Wednesday 10 July 2013, Alan Stern wrote:
> This isn't right.  There are USB host controllers that use PIO, not
> DMA.  The HAS_DMA dependency should go with the controller driver, not 
> the USB core.
> 
> On the other hand, the USB core does call various routines like 
> dma_unmap_single.  It ought to be possible to compile these calls even 
> when DMA isn't enabled.  That is, they should be defined as do-nothing 
> stubs.

The asm-generic/dma-mapping-broken.h file intentionally causes link
errors, but that could be changed.

The better approach in my mind would be to replace code like


	if (hcd->self.uses_dma)

with

	if (IS_ENABLED(CONFIG_HAS_DMA) && hcd->self.uses_dma) {

which will reliably cause that reference to be omitted from object code,
but not stop giving link errors for drivers that actually require
DMA.

	Arnd
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