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

From: Dan Williams
Date: Wed Feb 08 2017 - 11:25:19 EST

On Wed, Feb 8, 2017 at 12:57 AM, Anup Patel <anup.patel@xxxxxxxxxxxx> wrote:
> On Tue, Feb 7, 2017 at 11:46 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> On Tue, Feb 7, 2017 at 1:02 AM, Anup Patel <anup.patel@xxxxxxxxxxxx> wrote:
>>> On Tue, Feb 7, 2017 at 1:57 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>>>> On Tue, Feb 7, 2017 at 12:16 AM, Anup Patel <anup.patel@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>>>>> Reviewed-by: Scott Branden <scott.branden@xxxxxxxxxxxx>
>>>> 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.