Re: [RFC 1/1] drivers/dma/*: replace tasklets with workqueue

From: Allen Pais <apais@linux.microsoft.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Vinod Koul" <vkoul@kernel.org>,
	"Linus Walleij" <linus.walleij@linaro.org>,
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	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Škrabec" <jernej.skrabec@gmail.com>,
	"Samuel Holland" <samuel@sholland.org>,
	dmaengine@vger.kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 1/1] drivers/dma/*: replace tasklets with workqueue
Date: Tue, 31 May 2022 11:19:49 -0700	[thread overview]
Message-ID: <708F627F-0F7D-477F-BDF7-274424501DA0@linux.microsoft.com> (raw)
In-Reply-To: <CAK8P3a3vf_huJ5ysBvFDV=11E-=OxFGiDQ9ND04YKug8g6jV_A@mail.gmail.com>

> 
> [note: something is wrong with your email client, your previous reply appears to
> be in HTML]
> 
  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.
>> 
>> 
>>   I am not an expert in dma subsystem, but by using completion from
>> Hardirq context be a concern? Especially with latency.
> 
> 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. 

  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.

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