Re: [PATCH 2/9] dma: Convert from tasklet to BH workqueue

From: Vinod Koul <vkoul@kernel.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Allen Pais" <apais@linux.microsoft.com>,
	linux-kernel@vger.kernel.org, "Tejun Heo" <tj@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Hector Martin" <marcan@marcan.st>,
	"Sven Peter" <sven@svenpeter.dev>,
	"Florian Fainelli" <florian.fainelli@broadcom.com>,
	"Ray Jui" <rjui@broadcom.com>,
	"Scott Branden" <sbranden@broadcom.com>,
	"Paul Cercueil" <paul@crapouillou.net>,
	Eugeniy.Paltsev@synopsys.com,
	"Manivannan Sadhasivam" <manivannan.sadhasivam@linaro.org>,
	"Viresh Kumar" <vireshk@kernel.org>,
	"Frank Li" <Frank.Li@nxp.com>, "Leo Li" <leoyang.li@nxp.com>,
	zw@zh-kernel.org, "Zhou Wang" <wangzhou1@hisilicon.com>,
	haijie1@huawei.com, "Shawn Guo" <shawnguo@kernel.org>,
	"Sascha Hauer" <s.hauer@pengutronix.de>,
	"Sean Wang" <sean.wang@mediatek.com>,
	"Matthias Brugger" <matthias.bgg@gmail.com>,
	"AngeloGioacchino Del Regno"
	<angelogioacchino.delregno@collabora.com>,
	"Andreas Färber" <afaerber@suse.de>,
	logang@deltatee.com, "Daniel Mack" <daniel@zonque.org>,
	"Haojian Zhuang" <haojian.zhuang@gmail.com>,
	"Robert Jarzmik" <robert.jarzmik@free.fr>,
	"Bjorn Andersson" <andersson@kernel.org>,
	"Konrad Dybcio" <konrad.dybcio@linaro.org>,
	"Orson Zhai" <orsonzhai@gmail.com>,
	"Baolin Wang" <baolin.wang@linux.alibaba.com>,
	"Chunyan Zhang" <zhang.lyra@gmail.com>,
	"Patrice Chotard" <patrice.chotard@foss.st.com>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Skrabec" <jernej.skrabec@gmail.com>,
	peter.ujfalusi@gmail.com, "K. Y. Srinivasan" <kys@microsoft.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"Wei Liu" <wei.liu@kernel.org>,
	"Dexuan Cui" <decui@microsoft.com>,
	"Jassi Brar" <jassisinghbrar@gmail.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	maintainers@bluecherrydvr.com, aubin.constans@microchip.com,
	"Ulf Hansson" <ulf.hansson@linaro.org>,
	"Manuel Lauss" <manuel.lauss@gmail.com>,
	"Michał Mirosław" <mirq-linux@rere.qmqm.pl>,
	"jh80.chung" <jh80.chung@samsung.com>,
	oakad@yahoo.com,
	"Kunihiko Hayashi" <hayashi.kunihiko@socionext.com>,
	"Masami Hiramatsu" <mhiramat@kernel.org>,
	brucechang@via.com.tw, HaraldWelte@viatech.com, pierre@ossman.eu,
	duncan.sands@free.fr, "Alan Stern" <stern@rowland.harvard.edu>,
	"Oliver Neukum" <oneukum@suse.com>,
	openipmi-developer@lists.sourceforge.net,
	dmaengine@vger.kernel.org, asahi@lists.linux.dev,
	linux-arm-kernel@lists.infradead.org,
	linux-rpi-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
	imx@lists.linux.dev, linuxppc-dev@lists.ozlabs.org,
	linux-mediatek@lists.infradead.org,
	linux-actions@lists.infradead.org, linux-arm-msm@vger.kernel.org,
	linux-riscv@lists.infradead.org, linux-sunxi@lists.linux.dev,
	linux-tegra@vger.kernel.org, linux-hyperv@vger.kernel.org,
	linux-rdma@vger.kernel.org, linux-media@vger.kernel.org,
	"linux-mmc @ vger . kernel . org" <linux-mmc@vger.kernel.org>,
	Linux-OMAP <linux-omap@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	linux-s390@vger.kernel.org, Netdev <netdev@vger.kernel.org>,
	linux-usb@vger.kernel.org
Subject: Re: [PATCH 2/9] dma: Convert from tasklet to BH workqueue
Date: Fri, 29 Mar 2024 00:01:43 +0530	[thread overview]
Message-ID: <ZgW3j1qkLA-QU4iM@matsya> (raw)
In-Reply-To: <2e9257af-c123-406b-a189-eaebeecc1d71@app.fastmail.com>

On 28-03-24, 11:08, Arnd Bergmann wrote:
> On Thu, Mar 28, 2024, at 06:55, Vinod Koul wrote:
> > On 27-03-24, 16:03, Allen Pais wrote:
> >> The only generic interface to execute asynchronously in the BH context is
> >> tasklet; however, it's marked deprecated and has some design flaws. To
> >> replace tasklets, BH workqueue support was recently added. A BH workqueue
> >> behaves similarly to regular workqueues except that the queued work items
> >> are executed in the BH context.
> >
> > Thanks for conversion, am happy with BH alternative as it helps in
> > dmaengine where we need shortest possible time between tasklet and
> > interrupt handling to maximize dma performance
> 
> I still feel that we want something different for dmaengine,
> at least in the long run. As we have discussed in the past,
> the tasklet context in these drivers is what the callbacks
> from the dma client device is run in, and a lot of these probably
> want something other than tasklet context, e.g. just call
> complete() on a client-provided completion structure.
> 
> Instead of open-coding the use of the system_bh_wq in each
> dmaengine, how about we start with a custom WQ_BH
> specifically for the dmaengine subsystem and wrap them
> inside of another interface.
> 
> Since almost every driver associates the tasklet with the
> dma_chan, we could go one step further and add the
> work_queue structure directly into struct dma_chan,
> with the wrapper operating on the dma_chan rather than
> the work_queue.

I think that is very great idea. having this wrapped in dma_chan would
be very good way as well

Am not sure if Allen is up for it :-)

-- 
~Vinod