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

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Message-ID: <20110201161859.467676b0@katamari>
Date:	Tue, 1 Feb 2011 16:18:59 -0500
From:	Chuck Ebbert <cebbert@...hat.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	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 Tue, 1 Feb 2011 10:28:00 +0900
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> 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.
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