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

[Arnd Bergmann]

On Tue, May 31, 2022 at 8:19 PM Allen Pais <apais@xxxxxxxxxxxxxxxxxxx> wrote:
> >>> 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.

Right: around half the callbacks are a trivial version that does nothing
other than calling complete(), so these will benefit from skipping the
softirq, while the others still need the callback.

       Arnd