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

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Date:	Sat, 13 Jul 2013 03:42:55 +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

On 07/13/2013 03:08 AM, Sergei Shtylyov 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?

    Although, -1 won't give us the correct result in this case, it's 
more like + PAGE_SIZE - 1.

WBR, Sergei

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