Re: [RFC/RFT PATCH 3/5] scsi: Use dma_max_pfn(dev) helper for bounce_limitcalculations

[Sergei Shtylyov]

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@xxxxxxxxxxxxxxxx>
> > > > Cc: linux-scsi@xxxxxxxxxxxxxxx

> > > > Signed-off-by: Santosh Shilimkar <santosh.shilimkar@xxxxxx>
> > > > ---
> > > >    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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