linux-kernel - Re: [PATCH v2 2/5] async_tx: Handle DMA devices having support for fewer PQ coefficients

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Date:   Wed, 8 Feb 2017 08:24:59 -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 v2 2/5] async_tx: Handle DMA devices having support for
 fewer PQ coefficients

On Wed, Feb 8, 2017 at 12:57 AM, Anup Patel <anup.patel@...adcom.com> wrote:
> On Tue, Feb 7, 2017 at 11:46 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>> On Tue, Feb 7, 2017 at 1:02 AM, Anup Patel <anup.patel@...adcom.com> wrote:
>>> On Tue, Feb 7, 2017 at 1:57 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>>>> On Tue, Feb 7, 2017 at 12:16 AM, Anup Patel <anup.patel@...adcom.com> wrote:
>>>>> The DMAENGINE framework assumes that if PQ offload is supported by a
>>>>> DMA device then all 256 PQ coefficients are supported. This assumption
>>>>> does not hold anymore because we now have BCM-SBA-RAID offload engine
>>>>> which supports PQ offload with limited number of PQ coefficients.
>>>>>
>>>>> This patch extends async_tx APIs to handle DMA devices with support
>>>>> for fewer PQ coefficients.
>>>>>
>>>>> Signed-off-by: Anup Patel <anup.patel@...adcom.com>
>>>>> Reviewed-by: Scott Branden <scott.branden@...adcom.com>
>>>>
>>>> I don't like this approach. Define an interface for md to query the
>>>> offload engine once at the beginning of time. We should not be adding
>>>> any new extensions to async_tx.
>>>
>>> Even if we do capability checks in Linux MD, we still need a way
>>> for DMAENGINE drivers to advertise number of PQ coefficients
>>> handled by the HW.
>>>
>>> I agree capability checks should be done once in Linux MD but I don't
>>> see why this has to be part of BCM-SBA-RAID driver patches. We need
>>> separate patchsets to address limitations of async_tx framework.
>>
>> Right, separate enabling before we pile on new hardware support to a
>> known broken framework.
>
> Linux Async Tx not broken framework. The issue is:
> 1. Its not complete enough
> 2. Its not optimized for very high through-put offload engines

I'm not understanding your point. I'm nak'ing this change to add yet
more per-transaction capability checking to async_tx. I don't like the
DMA_HAS_FEWER_PQ_COEF flag, especially since it is equal to
DMA_HAS_PQ_CONTINUE. I'm not asking for all of async_tx's problems to
be fixed before this new hardware support, I'm simply saying we should
start the process of moving offload-engine capability checking to the
raid code.