linux-kernel - Re: [RFC/RFT PATCH 3/5] scsi: Use dma_max_pfn(dev) helper for bounce

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Message-ID: <51E08C6B.2080802@cogentembedded.com>
Date:	Sat, 13 Jul 2013 03:08:27 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Santosh Shilimkar <santosh.shilimkar@...com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-scsi@...r.kernel.org
Subject: Re: [RFC/RFT PATCH 3/5] scsi: Use dma_max_pfn(dev) helper for bounce_limit
 calculations

Hello.

On 07/13/2013 02:25 AM, Russell King - ARM Linux wrote:

>>> DMA bounce limit is the maximum direct DMA'able memory beyond which
>>> bounce buffers has to be used to perform dma operations. SCSI driver
>>> relies on dma_mask but its calculation is based on max_*pfn which
>>> don't have uniform meaning across architectures. So make use of
>>> dma_max_pfn() which is expected to return the DMAable maximum pfn
>>> value across architectures.

>>> Cc: Russell King <linux@....linux.org.uk>
>>> Cc: linux-scsi@...r.kernel.org

>>> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@...com>
>>> ---
>>>    drivers/scsi/scsi_lib.c |    2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)

>>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>>> index 86d5220..e8275fa 100644
>>> --- a/drivers/scsi/scsi_lib.c
>>> +++ b/drivers/scsi/scsi_lib.c
>>> @@ -1668,7 +1668,7 @@ u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
>>>
>>>    	host_dev = scsi_get_device(shost);
>>>    	if (host_dev && host_dev->dma_mask)
>>> -		bounce_limit = *host_dev->dma_mask;
>>> +		bounce_limit = dma_max_pfn(host_dev) << PAGE_SHIFT;

>>     You definitely forgot -1 here.

> Please explain your point.

    Previously, 'bounce_limit' would look like 0xffffffff (unless I'm 
mistaken), now it would look like 0xfffff000 which is hardly what we're 
looking for, no?

WBR, Sergei

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