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

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Date:	Thu, 11 Jul 2013 11:03:52 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
cc:	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: USB host support should depend on HAS_DMA

On Thu, 11 Jul 2013, Geert Uytterhoeven wrote:

> On Thu, Jul 11, 2013 at 3:01 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Thu, 11 Jul 2013, Arnd Bergmann wrote:
> >
> >> 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.
> >
> > How will it give link errors for drivers that require DMA?
> 
> It won't. Unless the host driver itself calls into the DMA API, too
> (are there any that don't?).

To my knowledge, all the host controller drivers which use DMA _do_
call functions in the DMA API.  So they would still get link errors,
even though the USB core wouldn't.

Therefore adding the appropriate HAS_DMA dependencies should be 
straightforward: Try to build all the drivers and see which ones fail 
to link.

Alan Stern

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