linux-kernel - Re: [PATCH 09/12] tegra20-apb-dma: remove useless use of lock

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Message-ID: <CAHp75Vfib7ogMcPp83u3=EfuzzSzZyRGGw8W1=m5qeWtcYp3Hw@mail.gmail.com>
Date:	Wed, 29 May 2013 16:42:56 +0300
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Laxman Dewangan <ldewangan@...dia.com>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	Stephen Warren <swarren@...dotorg.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 09/12] tegra20-apb-dma: remove useless use of lock

On Wed, May 29, 2013 at 1:56 PM, Laxman Dewangan <ldewangan@...dia.com> wrote:
> On Monday 27 May 2013 05:44 PM, Andy Shevchenko wrote:
>>
>> Accordingly to dma_cookie_status() description locking is not required.
>>
> I think we need lock here:
> From isr handler, we call dma_cookie_complete() which is in spin-locked.
> This function updates  tx->chan->completed_cookie = tx->cookie;
> In tegra_dma_tx_status(), we check for dma_cookie_status() which access the
> chan->completed_cookie; and it decides status based on this
>
> As the access of  chan->completed_cookie  are from different context, we
> need this locking.

You need to have a consistent data in the cookies. This is guaranteed
by memory barrier if I got it correctly.

-- 
With Best Regards,
Andy Shevchenko
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