linux-kernel - Re: [PATCH 2/5] mtd: nand: omap2: Start dma request before enabling prefetch

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Date:	Wed, 14 Oct 2015 08:45:22 -0500
From:	"Franklin S Cooper Jr." <fcooper@...com>
To:	Roger Quadros <rogerq@...com>, <linux-kernel@...r.kernel.org>,
	<devicetree@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<linux-mtd@...ts.infradead.org>, <nsekhar@...com>,
	<computersforpeace@...il.com>, <dwmw2@...radead.org>,
	<tony@...mide.com>
Subject: Re: [PATCH 2/5] mtd: nand: omap2: Start dma request before enabling
 prefetch



On 10/14/2015 06:41 AM, Roger Quadros wrote:
> On 13/10/15 04:38, Franklin S Cooper Jr wrote:
>> The prefetch engine sends a dma request once a FIFO threshold has
>> been met. No other requests are received until the previous request
>> is handled.
>>
>> Starting an edma transfer (dma_async_issue_pending) results in any
>> previous event for the dma channel to be cleared. Therefore, starting
>> the prefetch engine before initiating the dma transfer may result in
>> the prefetch triggering a dma request but instead of it being handled
>> it can end up being cleared. This will result in a hang since the code
>> will continue to wait for the dma request to complete.
>>
>> By initiating the dma request before enabling the prefetch engine this
>> race condition is avoided and no dma request are missed/cleared.
>>
>> Signed-off-by: Franklin S Cooper Jr <fcooper@...com>
>> ---
>>  drivers/mtd/nand/omap2.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/mtd/nand/omap2.c b/drivers/mtd/nand/omap2.c
>> index 957c32f..94d11de 100644
>> --- a/drivers/mtd/nand/omap2.c
>> +++ b/drivers/mtd/nand/omap2.c
>> @@ -509,6 +509,9 @@ static inline int omap_nand_dma_transfer(struct mtd_info *mtd, void *addr,
>>  	tx->callback_param = &info->comp;
>>  	dmaengine_submit(tx);
>>  
>> +	init_completion(&info->comp);
>> +	dma_async_issue_pending(info->dma);
>> +
>>  	/*  configure and start prefetch transfer */
>>  	ret = omap_prefetch_enable(info->gpmc_cs,
>>  		PREFETCH_FIFOTHRESHOLD_MAX, 0x1, len, is_write, info);
>> @@ -516,9 +519,6 @@ static inline int omap_nand_dma_transfer(struct mtd_info *mtd, void *addr,
>>  		/* PFPW engine is busy, use cpu copy method */
>>  		goto out_copy_unmap;
>>  
>> -	init_completion(&info->comp);
>> -	dma_async_issue_pending(info->dma);
>> -
>>  	/* setup and start DMA using dma_addr */
> Is the above comment misplaced after this change?
Yup you right.
>
>>  	wait_for_completion(&info->comp);
>>  	tim = 0;
>>
> cheers,
> -roger

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