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

[Anup Patel]

On Wed, Feb 8, 2017 at 9:54 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> 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.

The DMA_HAS_FEWER_PQ_COEF is not equal to
DMA_HAS_PQ_CONTINUE.

I will try to drop this patch and take care of unsupported PQ
coefficients in BCM-SBA-RAID driver itself even if this means
doing some computations in BCM-SBA-RAID driver itself.

Regards,
Anup