linux-kernel - Re: [PATCH] dma: Revert "dmaengine: mic_x100: add missing spin

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Message-ID: <20160106101119.GC11778@localhost>
Date:	Wed, 6 Jan 2016 15:41:19 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Ashutosh Dixit <ashutosh.dixit@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"Dutt, Sudeep" <sudeep.dutt@...el.com>,
	"Rao, Nikhil" <nikhil.rao@...el.com>,
	Siva Yerramreddy <yshivakrishna@...il.com>,
	Saurabh Sengar <saurabh.truth@...il.com>
Subject: Re: [PATCH] dma: Revert "dmaengine: mic_x100: add missing
 spin_unlock"

On Mon, Jan 04, 2016 at 05:40:39PM -0500, Ashutosh Dixit wrote:
> On Sun, Jan 03 2016 at 10:35:26 PM, "Koul, Vinod" <vinod.koul@...el.com> wrote:
> > On Tue, Dec 22, 2015 at 07:35:23PM -0800, Ashutosh Dixit wrote:
> >> This reverts commit e958e079e254 ("dmaengine: mic_x100: add missing
> >> spin_unlock").
> >>
> >> The above patch is incorrect. There is nothing wrong with the original
> >> code. The spin_lock is acquired in the "prep" functions and released
> >> in "submit".
> >
> > And going by dmaengine sematics, I do not think that is entrely right.
> >
> > A user may choose to prepare multiple desciptors and then sumbit later,
> > looking at code I do not see how that will work.
> >
> > Please fix that
> 
> The mic_x100_dma driver still allows a client to prepare and submit
> multiple descriptors and triggers the hardware only when issue_pending
> is called (or a threshold is exceeded). Identical coding patterns exist
> in the IOAT dma driver, on which the mic_x100_dma driver is based.
> 
> Further, mic_x100 dma channels are private and used only by other MIC
> drivers such as SCIF (drivers/misc/mic/scif). These drivers obviously
> alternate prep and submit calls as required by mic_x100_dma. We do not
> envisage a wider use of the mic_x100_dma driver at this point.

The whole point of using an API is to create standard usages, future clients
can come up and your argument is not future proof
> 
> A change such as allowing multiple prep's before a submit requires large
> scale changes in the driver. In the absence of a real use case, there is
> no plan to make such changes at present. At this point we only want the
> driver to be restored to its previously functional state for the 4.4
> kernel. The patch in question results in system lockups so it is a
> serious v4.4 regression.

I will revert it but please fix the driver..

-- 
~Vinod
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