[1] The gpmi uses the nand_command_lp to issue the commands to NAND chips.
The gpmi issues a DMA operation with gpmi_cmd_ctrl when it handles
a NAND_CMD_NONE control command. So when we read a page(NAND_CMD_READ0)
from the NAND, we may send two DMA operations back-to-back.
If we do not serialize the two DMA operations, we will meet a bug when
1.1) we enable CONFIG_DMA_API_DEBUG, CONFIG_DMADEVICES_DEBUG,
and CONFIG_DEBUG_SG.
1.2) Use the following commands in an UART console and a SSH console:
cmd 1: while true;do dd if=/dev/mtd0 of=/dev/null;done
cmd 1: while true;do dd if=/dev/mmcblk0 of=/dev/null;done
The kernel log shows below:
-----------------------------------------------------------------
kernel BUG at lib/scatterlist.c:28!
Unable to handle kernel NULL pointer dereference at virtual address 00000000
.........................
[<80044a0c>] (__bug+0x18/0x24) from [<80249b74>] (sg_next+0x48/0x4c)
[<80249b74>] (sg_next+0x48/0x4c) from [<80255398>] (debug_dma_unmap_sg+0x170/0x1a4)
[<80255398>] (debug_dma_unmap_sg+0x170/0x1a4) from [<8004af58>] (dma_unmap_sg+0x14/0x6c)
[<8004af58>] (dma_unmap_sg+0x14/0x6c) from [<8027e594>] (mxs_dma_tasklet+0x18/0x1c)
[<8027e594>] (mxs_dma_tasklet+0x18/0x1c) from [<8007d444>] (tasklet_action+0x114/0x164)
-----------------------------------------------------------------
1.3) Assume the two DMA operations is X (first) and Y (second).
The root cause of the bug:
Assume process P issues DMA X, and sleep on the completion
@this->dma_done. X's tasklet callback is dma_irq_callback. It firstly
wake up the process sleeping on the completion @this->dma_done,
and then trid to unmap the scatterlist S. The waked process P will
issue Y in another ARM core. Y initializes S->sg_magic to zero
with sg_init_one(), while dma_irq_callback is unmapping S at the same
time.
See the diagram:
ARM core 0 | ARM core 1
-------------------------------------------------------------
(P issues DMA X, then sleep) --> |
|
(X's tasklet wakes P) --> |
|
| <-- (P begin to issue DMA Y)
|
(X's tasklet unmap the |
scatterlist S with dma_unmap_sg) --> | <-- (Y calls sg_init_one() to init
| scatterlist S)
|
[2] This patch serialize both the X and Y in the following way:
Unmap the DMA scatterlist S firstly, and wake up the process at the end
of the DMA callback, in such a way, Y will be executed after X.
After this patch:
ARM core 0 | ARM core 1
-------------------------------------------------------------
(P issues DMA X, then sleep) --> |
|
(X's tasklet unmap the |
scatterlist S with dma_unmap_sg) --> |
|
(X's tasklet wakes P) --> |
|
| <-- (P begin to issue DMA Y)
|
| <-- (Y calls sg_init_one() to init
| scatterlist S)
|
Cc: stable@vger.kernel.org # 3.2
Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
7b3d2fb920