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

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Message-ID: <CAALAos-txDCs3QZ4HGBNicOD8t49NPT6E8RpjVMccMn1D-UTgQ@mail.gmail.com>
Date:   Wed, 15 Feb 2017 12:33:06 +0530
From:   Anup Patel <anup.patel@...adcom.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Vinod Koul <vinod.koul@...el.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Jon Mason <jonmason@...adcom.com>,
        Rob Rice <rob.rice@...adcom.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        Device Tree <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-crypto@...r.kernel.org,
        linux-raid <linux-raid@...r.kernel.org>
Subject: Re: [PATCH v4 3/4] dmaengine: Add Broadcom SBA RAID driver

On Wed, Feb 15, 2017 at 12:13 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Tue, Feb 14, 2017 at 10:25 PM, Anup Patel <anup.patel@...adcom.com> wrote:
>> On Tue, Feb 14, 2017 at 10:04 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>>> On Mon, Feb 13, 2017 at 10:51 PM, Anup Patel <anup.patel@...adcom.com> 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@...adcom.com>
>>>> Reviewed-by: Ray Jui <ray.jui@...adcom.com>
>>>> ---
>>>>  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".
>
> No, fix the infrastructure. Do not put local hack in your driver for
> this global problem [1].

There is no hack in the driver. We need
ASYNC_TX_ENABLE_CHANNEL_SWITCH
based on current state of dmaengine framework.

The framework should be fixed as separate patchset.

We have other RAID drivers such as xgene-dma and
mv_xor_v2 who also require
ASYNC_TX_ENABLE_CHANNEL_SWITCH due
to same reason.

Fixing the framework and improving framework is
a ongoing process. I don't see why that should
stop this patchset.

Regards,
Anup