Re: [PATCH 4.19 28/37] dmaengine: dmatest: Fix iteration non-stop logic

Pavel Machek
2020-05-05T12:32:03Z

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Hi!

> So, to the point, the conditional of checking the thread to be stopped being
> first part of conjunction logic prevents to check iterations. Thus, we have to
> always check both conditions to be able to stop after given
> iterations.

I ... don't understand. AFAICT the code is equivalent. Both && and ||
operators permit "short" execution... but second part of expression
has no sideeffects, so...

> @@ -567,8 +567,8 @@ static int dmatest_func(void *data)
> flags = DMA_CTRL_ACK | DMA_PREP_INTERRUPT;
>
> ktime = ktime_get();
> - while (!kthread_should_stop()
> - && !(params->iterations && total_tests >= params->iterations)) {
> + while (!(kthread_should_stop() ||
> + (params->iterations && total_tests >= params->iterations))) {
> struct dma_async_tx_descriptor *tx = NULL;
> struct dmaengine_unmap_data *um;
> dma_addr_t *dsts;

let a = kthread_should_stop()
let b = (params->iterations && total_tests >= params->iterations)

You are changing !a & !b into !(a | b). But that's equivalent
expression. I hate to admit, but I had to draw truth table to prove
that.

!a & !b 0 0 -> 1
else -> 0

!(a | b) 0 0 -> 1
else -> 0

What am I missing?

Best regards,
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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