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

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Date:   Tue, 14 Feb 2017 22:43:05 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Anup Patel <anup.patel@...adcom.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 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].

[1]: https://lwn.net/Articles/443531/