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

From: Dan Williams
Date: Thu Feb 09 2017 - 11:45:09 EST

On Thu, Feb 9, 2017 at 1:29 AM, Anup Patel <anup.patel@xxxxxxxxxxxx> wrote:
> 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

#define DMA_HAS_PQ_CONTINUE (1 << 15
#define DMA_HAS_FEWER_PQ_COEF (1 << 15)

> 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.

That should be nak'd as well, please do capability detection in a
routine that is common to all raid engines.