linux-kernel - Re: [PATCH 2/3] dma: add __must_check annotation for dmaengine

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Message-ID: <55C9EBD5.3090203@linutronix.de>
Date:	Tue, 11 Aug 2015 14:34:29 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Vinod Koul <vinod.koul@...el.com>
CC:	peter@...leysoftware.com, Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	nsekhar@...com, linux-omap@...r.kernel.org,
	linux-serial@...r.kernel.org, john.ogness@...utronix.de,
	Peter Ujfalusi <peter.ujfalusi@...com>
Subject: Re: [PATCH 2/3] dma: add __must_check annotation for dmaengine_pause()

On 08/11/2015 12:06 PM, Russell King - ARM Linux wrote:
> I think what people need to learn is that an API in the kernel which
> returns an int _can_ fail - it returns an int so it _can_ return an
> error code.  If it _can_ return an error code, there _will_ be
> implementations which _do_.
> 
> If you don't check the return code, either your code doesn't care whether
> the function was successful or not, or you're playing with fire.  This is
> a prime example of playing with fire.
> 
> Let's leave the crappy userspace laziness with regard to error checking
> to userspace, and keep it out of the kernel.
> 
> Yes, the DMA engine capabilities may not be sufficient to describe every
> detail of DMA engines, but that's absolutely no reason to skimp on error
> checking.  Had there been some kind of error checking at the site, this
> problem would have been spotted before the 8250-omap driver was merged.

Let me disable RX-DMA in 8250-omap code and push that stable. Then we
won't need a special annotation for pause support because it remains
off and is currently about one user. I browsed each driver in
drivers/dma each one which does support pause supports it and all of
them implement it unconditionally (ipu_idmac grabs a mutex first but
this is another story).
Adding error checking to 8250-omap like I have it in #1 and disabling
RX-DMA in case pause fails looks be reasonable since there is nothing
else that can be done I guess.
Once we have the missing piece in omap-dma the RX-DMA can be enabled in
8250-omap.
Does this sound like a plan we can agree on?

Sebastian
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