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

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Message-ID: <CAOMdWSKY9D75FM3bswUfXn2o7bGtrei3G5kLt6JdcdOPDXaG8g@mail.gmail.com>
Date: Thu, 28 Mar 2024 12:39:30 -0700
From: Allen <allen.lkml@...il.com>
To: Vinod Koul <vkoul@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>, Allen Pais <apais@...ux.microsoft.com>, 
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Subject: Re: [PATCH 2/9] dma: Convert from tasklet to BH workqueue

> > >> 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 :-)

 Thanks Arnd, I know we did speak about this at LPC. I did start
working on using completion. I dropped it as I thought it would
be easier to move to workqueues.

Vinod, I would like to give this a shot and put out a RFC, I would
really appreciate review and feedback.

Thanks,
Allen

>
> --
> ~Vinod
>