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Date:	Mon, 16 May 2016 10:49:04 +0530
From:	"Sricharan" <sricharan@...eaurora.org>
To:	"'Wolfram Sang'" <wsa@...-dreams.de>
Cc:	<devicetree@...r.kernel.org>, <architt@...eaurora.org>,
	<linux-arm-msm@...r.kernel.org>, <ntelkar@...eaurora.org>,
	<galak@...eaurora.org>, <linux-kernel@...r.kernel.org>,
	<andy.gross@...aro.org>, <linux-i2c@...r.kernel.org>,
	<iivanov@...sol.com>, <agross@...eaurora.org>,
	<dmaengine@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <nkaje@...eaurora.org>
Subject: RE: [PATCH V2 1/2] drivers: i2c: qup: Fix broken dma when CONFIG_DEBUG_SG is enabled

Hi,
> > With CONFIG_DEBUG_SG is enabled and when dma mode is used, below
> dump
> > is seen,
> >
> > ------------[ cut here ]------------
> > kernel BUG at include/linux/scatterlist.h:140!
> > Internal error: Oops - BUG: 0 [#1] PREEMPT SMP Modules linked in:
> > CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.4.0-00459-g9f087b9-dirty
> > #7 Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
> > task: ffffffc036868000 ti: ffffffc036870000 task.ti: ffffffc036870000
> > PC is at qup_sg_set_buf.isra.13+0x138/0x154
> > LR is at qup_sg_set_buf.isra.13+0x50/0x154 pc : [<ffffffc0005a0ed8>]
> > lr : [<ffffffc0005a0df0>] pstate: 60000145 sp : ffffffc0368735c0
> > x29: ffffffc0368735c0 x28: ffffffc036873752
> > x27: ffffffc035233018 x26: ffffffc000c4e000
> > x25: 0000000000000000 x24: 0000000000000004
> > x23: 0000000000000000 x22: ffffffc035233668
> > x21: ffffff80004e3000 x20: ffffffc0352e0018
> > x19: 0000004000000000 x18: 0000000000000028
> > x17: 0000000000000004 x16: ffffffc0017a39c8
> > x15: 0000000000001cdf x14: ffffffc0019929d8
> > x13: ffffffc0352e0018 x12: 0000000000000000
> > x11: 0000000000000001 x10: 0000000000000001
> > x9 : ffffffc0012b2d70 x8 : ffffff80004e3000
> > x7 : 0000000000000018 x6 : 0000000030000000
> > x5 : ffffffc00199f018 x4 : ffffffc035233018
> > x3 : 0000000000000004 x2 : 00000000c0000000
> > x1 : 0000000000000003 x0 : 0000000000000000
> >
> > Process swapper/0 (pid: 1, stack limit = 0xffffffc036870020)
> > Stack: (0xffffffc0368735c0 to 0xffffffc036874000)
> >
> > Change allocation of sg buffers from dma_coherent memory to kzalloc to
> > fix the issue.
> 
> This description describes what you do. But not why it is the correct
solution
> to the OOPS. The OOPS  neither describes it. Please add some more
> explanation.
> 
   Ok,will describe it more. The reason it oops is sg_set_bug expects that
the
   buf parameter passed in should be a from the lowmem and a valid
pageframe.
  This is not true for pages from dma_alloc_coherent which can be carveouts,
hence
  the check fails. Allocating buffers using kzalloc fixes the issue.

Regards,
 Sricharan
  

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