linux-kernel - Re: b44 driver causes panic when using swiotlb

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Date:	Wed, 2 Feb 2011 09:09:22 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	fujita.tomonori@....ntt.co.jp
Cc:	cebbert@...hat.com, hancockrwd@...il.com, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org, dwmw2@...radead.org
Subject: Re: b44 driver causes panic when using swiotlb

On Wed, 2 Feb 2011 09:06:34 +0900
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:

> On Tue, 1 Feb 2011 16:18:59 -0500
> Chuck Ebbert <cebbert@...hat.com> wrote:
> 
> > > swiotlb allocates the bounce buffer when a system boots up. We can't
> > > allocate much in GFP_DMA. swiotlb uses somewhere under 4GB. So it
> > > can't help devices that have odd dma_mask (that is, except for 4GB).
> > > 
> > > Unfortunately, Such device needs to do own custom bouncing or needs
> > > their subsystem to does that.
> > 
> > I think we're chasing the wrong problem here.
> > 
> > swiotlb uses alloc_bootmem_low_pages() to try to get buffers as low
> > in memory as possible. I asked someone who is hitting this bug to
> > try 2.6.36 and he reports the buffers really are low there:
> > 
> > 2.6.36:  5c00000
> > 2.6.37: db600000
> > 
> > So something happened very early in the 2.6.37-rc cycle that changed
> > this behavior. I tried looking at the bootmem code but could not see
> > the problem. The only related option I could find in .config was this:
> > 
> > # CONFIG_NO_BOOTMEM is not set
> > 
> > It was set this way in both .36 and .37.
> 
> I don't think this matters because we can't guarantee that swiotlb
> can't allocate ZONE_DMA.

Oops, I meant that, we can't guarantee that swiotlb allocates
ZONE_DMA.

> Obviously, the default swiotlb bouncing
> buffer size, 64MB, is too large for ZONE_DMA.
> 
> If you are lucky, you sometimes can allocate lower memory if swiotlb
> allocates lower memory at the boot time. But you can't assume
> that. swiotlb isn't designed to handle such.
> 
> Can you try the patch that I sent you? As I wrote, b44 has the own
> bouncing mechanism, so we should be fine if swiotlb returns an
> appropriate error instead of just panicking.
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