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

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Message-ID: <51A5DEDC.4040606@nvidia.com>
Date:	Wed, 29 May 2013 16:26:28 +0530
From:	Laxman Dewangan <ldewangan@...dia.com>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:	"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 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.

But did not get why it is documented as locking is not require if shared 
variable is getting changed/access from different context simultaneously.

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