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Subject: Re: [RFC 1/1] drivers/dma/*: replace tasklets with workqueue
From:   Allen Pais <apais@linux.microsoft.com>
In-Reply-To: <CAK8P3a3vf_huJ5ysBvFDV=11E-=OxFGiDQ9ND04YKug8g6jV_A@mail.gmail.com>
Date:   Tue, 31 May 2022 11:19:49 -0700
Cc:     Vinod Koul <vkoul@kernel.org>,
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To:     Arnd Bergmann <arnd@arndb.de>
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>=20
> [note: something is wrong with your email client, your previous reply =
appears to
> be in HTML]
>=20
  Sorry, fixed it.

>>> That is a good idea, lot of drivers are waiting for completion which =
can
>>> be signalled from hardirq, this would also reduce the hops we have =
and
>>> help improve latency a bit. On the downside, some controllers =
provide
>>> error information, which would need to be dealt with.
>>=20
>>=20
>>   I am not an expert in dma subsystem, but by using completion from
>> Hardirq context be a concern? Especially with latency.
>=20
> I don't see how: to the task waiting for the completion, there should
> be no difference, and for the irq handler sending it, it just avoids
> a few cycles going into softirq context.

  Thanks for clarification.=20

  If I have understood it correctly, your suggestion is to move the =
current
Callback mechanism out to dmaengine as a generic helper function
And introduce completion in dma_async_tx_descriptor to handle what
Tasklets currently do.

Thanks.

>=20
>>> Yes that would be a very reasonable mechanism, thanks for the
>>> suggestions.
>>=20
>>  I have started working on the idea of global softirq. A RFC should =
be ready
>> For review soon.
>=20
> Ok, thanks!
>=20
>       Arnd