Re: [PATCH v4 3/4] dmaengine: Add Broadcom SBA RAID driver

[Anup Patel]

On Tue, Feb 14, 2017 at 10:04 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Mon, Feb 13, 2017 at 10:51 PM, Anup Patel <anup.patel@xxxxxxxxxxxx> wrote:
>> The Broadcom stream buffer accelerator (SBA) provides offloading
>> capabilities for RAID operations. This SBA offload engine is
>> accessible via Broadcom SoC specific ring manager.
>>
>> This patch adds Broadcom SBA RAID driver which provides one
>> DMA device with RAID capabilities using one or more Broadcom
>> SoC specific ring manager channels. The SBA RAID driver in its
>> current shape implements memcpy, xor, and pq operations.
>>
>> Signed-off-by: Anup Patel <anup.patel@xxxxxxxxxxxx>
>> Reviewed-by: Ray Jui <ray.jui@xxxxxxxxxxxx>
>> ---
>>  drivers/dma/Kconfig        |   13 +
>>  drivers/dma/Makefile       |    1 +
>>  drivers/dma/bcm-sba-raid.c | 1694 ++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 1708 insertions(+)
>>  create mode 100644 drivers/dma/bcm-sba-raid.c
>>
>> diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
>> index 263495d..bf8fb84 100644
>> --- a/drivers/dma/Kconfig
>> +++ b/drivers/dma/Kconfig
>> @@ -99,6 +99,19 @@ config AXI_DMAC
>>           controller is often used in Analog Device's reference designs for FPGA
>>           platforms.
>>
>> +config BCM_SBA_RAID
>> +       tristate "Broadcom SBA RAID engine support"
>> +       depends on (ARM64 && MAILBOX && RAID6_PQ) || COMPILE_TEST
>> +       select DMA_ENGINE
>> +       select DMA_ENGINE_RAID
>> +       select ASYNC_TX_ENABLE_CHANNEL_SWITCH
>
> I thought you agreed to drop this. Its usage is broken.

If ASYNC_TX_ENABLE_CHANNEL_SWITCH is not selected
then async_dma_find_channel() will only try to find channel
with DMA_ASYNC_TX capability.

The DMA_ASYNC_TX capability is set by
dma_async_device_register() when all Async Tx
capabilities are supported by a DMA devices namely
DMA_INTERRUPT, DMA_MEMCPY, DMA_XOR,
DMA_XOR_VAL, DMA_PQ, and DMA_PQ_VAL.

We only support DMA_MEMCPY, DMA_XOR, and
DMA_PQ capabilities in BCM-SBA-RAID driver so
DMA_ASYNC_TX capability is never set for the
DMA device registered by BCM-SBA-RAID driver.

Due to above, if ASYNC_TX_ENABLE_CHANNEL_SWITCH
is not selected then Async Tx APIs fail to find DMA
channel provided by BCM-SBA-RAID hence the
option ASYNC_TX_ENABLE_CHANNEL_SWITCH is
required for BCM-SBA-RAID.

The DMA mappings are violated by channel switching
only if we switch form DMA channel A to DMA channel
B and both these DMA channels have different underlying
"struct device". In most of the cases DMA mappings
are not violated because DMA channels having
Async Tx capabilities are provided using same
underlying "struct device".

Regards,
Anup