netdev - Re: Softirq priority inversion from "softirq: reduce latencies"

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Message-ID: <56DDA2BE.7050800@linutronix.de>
Date:	Mon, 7 Mar 2016 16:48:14 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Hurley <peter@...leysoftware.com>,
	Eric Dumazet <edumazet@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
	dmaengine@...r.kernel.org, John Ogness <john.ogness@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"

On 02/27/2016 07:19 PM, Peter Hurley wrote:
> Hi Eric,

Hi Peter,

> Because both the uart driver (omap8250) and the dmaengine driver
> (edma) were (relatively) new, we assumed there was some race between
> starting a new rx DMA and processing the previous one.

Now after digesting the whole thread. I complained about this a long
while ago. After you start RX-DMA the DMA-engine is not programmed
immediately but deferred into softirq/tasklet. This is not the case for
continuous DMA transfer - those are programmed right away.

I don't remember that I found a reason why this simple programming has
to be deferred and can't happen immediately like it is the case for the
continuous DMA transfers. So I skipped that. RX-DMA in UART was working
well but for some reason omap's MMC-card driver refused to work.

Sebastian