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

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Date: Fri, 29 Mar 2024 00:01:43 +0530
From: Vinod Koul <vkoul@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Allen Pais <apais@...ux.microsoft.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH 2/9] dma: Convert from tasklet to BH workqueue

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