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

From: Allen Pais <apais@linux.microsoft.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "David Laight" <David.Laight@aculab.com>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Vincent Guittot" <vincent.guittot@linaro.org>,
	"olivier.dautricourt@orolia.com" <olivier.dautricourt@orolia.com>,
	"Stefan Roese" <sr@denx.de>, "Vinod Koul" <vkoul@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"linux-hardening@vger.kernel.org"
	<linux-hardening@vger.kernel.org>,
	"Ludovic Desroches" <ludovic.desroches@microchip.com>,
	"Tudor Ambarus" <tudor.ambarus@microchip.com>,
	"Florian Fainelli" <f.fainelli@gmail.com>,
	"Ray Jui" <rjui@broadcom.com>,
	"Scott Branden" <sbranden@broadcom.com>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@broadcom.com>,
	"Nicolas Saenz Julienne" <nsaenz@kernel.org>,
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	"Viresh Kumar" <vireshk@kernel.org>,
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	"Manivannan Sadhasivam" <mani@kernel.org>,
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	"Patrice CHOTARD" <patrice.chotard@foss.st.com>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Škrabec" <jernej.skrabec@gmail.com>,
	"Samuel Holland" <samuel@sholland.org>,
	"dmaengine@vger.kernel.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: Wed, 25 May 2022 11:04:11 -0700	[thread overview]
Message-ID: <45597969-0419-4CEE-B80C-4D2917943837@linux.microsoft.com> (raw)
In-Reply-To: <CAK8P3a0mOEMW3N7HB7zTAbqKJMu_RusivjxwuU7_E+O1vGHOEw@mail.gmail.com>

>> 
>> Thank you Linus, Arnd, Vincent and David for the feedback.
>> 
>> This is a lot more than a just conversion of API’s. I am in the process
>> Of replacing tasklets with threaded irq’s and hopefully that should be
>> A better solution than using workqueues.
> 
> I don't think that is much better for the case of the DMA engine
> callbacks than the work queue, the problem I pointed out here
> is scheduling into task context, which may be too expensive
> in some cases, but whether it is or not depends on the slave
> driver, not the dmaengine driver.

Fair point. Deferring all callbacks to task context is not the ideal
Way forward. I will work on the approach you shared,

>1. add helper functions to call the callback functions from a
>tasklet locally defined in drivers/dma/dmaengine.c to allow
>deferring it from hardirq context

>2. Change all tasklets that are not part of the callback
>mechanism to work queue functions, I only see
>xilinx_dpdma_chan_err_task in the patch, but there
>may be more

>3. change all drivers to move their custom tasklets back into
>hardirq context and instead call the new helper for deferring
>the callback.

>4. Extend the dmaengine callback API to let slave drivers
>pick hardirq, tasklet or task context for the callback.
>task context can mean either a workqueue, or a threaded
>IRQ here, with the default remaining the tasklet version.

>5. Change slave drivers to pick either hardirq or task context
>depending on their requirements

>6. Remove the tasklet version.

Thanks.

> 
> Even if all the slave drivers are better off using task context
> (threaded irq or workqueue), you need to look at the slave
> drivers first before you can convert the dmaengine drivers.
> 