linux-kernel - Re: [PATCH 6/6] dma/imx-sdma: check whether event_id0 < 32 when set event

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Date:	Wed, 11 Jan 2012 20:35:57 +0800
From:	Richard Zhao <richard.zhao@...aro.org>
To:	Eric Miao <eric.miao@...aro.org>
Cc:	Richard Zhao <richard.zhao@...escale.com>,
	Shawn Guo <shawn.guo@...aro.org>, patches@...aro.org,
	vinod.koul@...el.com, linux-kernel@...r.kernel.org,
	kernel@...gutronix.de, dan.j.williams@...el.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 6/6] dma/imx-sdma: check whether event_id0 < 32 when set
 event_mask

On Wed, Jan 11, 2012 at 02:37:08PM +0800, Eric Miao wrote:
> On Wed, Jan 11, 2012 at 9:47 AM, Richard Zhao
> <richard.zhao@...escale.com> wrote:
> > On Wed, Jan 11, 2012 at 08:53:23AM +0800, Richard Zhao wrote:
> >> On Tue, Jan 10, 2012 at 11:38:39PM +0800, Shawn Guo wrote:
> >> > On Tue, Jan 10, 2012 at 10:29:42PM +0800, Richard Zhao wrote:
> >> > > On Tue, Jan 10, 2012 at 10:20:10PM +0800, Shawn Guo wrote:
> >> > > > On Tue, Jan 10, 2012 at 03:01:50PM +0800, Richard Zhao wrote:
> >> > > > > Signed-off-by: Richard Zhao <richard.zhao@...aro.org>
> >> > > > > ---
> >> > > >
> >> > > > I think it deserves a sensible commit message explaining why the patch
> >> > > > is needed.
> >> > > If event_id0 < 32, 1 << (sdmac->event_id0 - 32) is not zero.
> >> This meant to make you clear about the patch. I'll add it in commit
> >> message.
> > unsigned int t = 31;
> > printf("%d %08x\n", t, 1 << (t-32));
> >
> > I test above code on both x86 and arm. They shows different results.
> > x86: 31 80000000
> > arm: 31 00000000
> >
> > I think we still need this patch. we shoud not let it depends on gcc's
> > behavior.
> >
> > Thanks
> > Richard
> >> > >
> >> > My point is you may explain the exact problem you are seeing without
> >> > this patch
> >> The kernel don't have event_id < 32 case yet. I found the bug when
> >> I review the code.
> >> > and how the patch helps here.  In general, doing so would
> >> > win a warm feeling from reviewers much more easily than leaving the
> >> > commit message empty there.
> >> I understand your point that comment as much as possible.
> >>
> 
> Shawn,
> 
> I think Richard has made the issue quite clear here, the original
> code does seem to have some problems even to me, who do not
> understand the very details of the SDMA:
> 
> -                       sdmac->event_mask0 = 1 << sdmac->event_id0;
> -                       sdmac->event_mask1 = 1 << (sdmac->event_id0 - 32);
> 
> 1. if sdmac->event_id0 >= 32, which will cause event_mask0 to be incorrect
> 2. if sdmac->event_id < 32, sdmac->event_mask1 will be incorrect
> 
> An alternate way is to use the standard bit operations:
> 
> struct sdma_channel {
> 
> 	...
> 
> 	unsigned long event_mask[2];
> 
> 	...
> };
> 
> 	set_bit(sdmac->event_id0, event_mask);
> 
> Which avoids branch instructions and add a bit protection for the operation
> to be atomic enough (event_mask0/1 won't be inconsistent).
It's a good idea.

Thanks
Richard
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