Re: [PATCH 1/1] ARM: dma: fix dma_max_pfn()

[Russell King - ARM Linux]

On Thu, Aug 18, 2016 at 09:55:55AM -0700, Santosh Shilimkar wrote:
> Hi Russell,
> 
> On 8/18/2016 7:24 AM, Russell King - ARM Linux wrote:
> >On Wed, Aug 17, 2016 at 03:05:17PM +0300, Roger Quadros wrote:
> >>Since commit 6ce0d2001692 ("ARM: dma: Use dma_pfn_offset for dma address translation"),
> >>dma_to_pfn() already returns the PFN with the physical memory start offset
> >>so we don't need to add it again.
> >>
> >>This fixes USB mass storage lock-up problem on systems that can't do DMA
> >>over the entire physical memory range (e.g.) Keystone 2 systems with 4GB RAM
> >>can only do DMA over the first 2GB. [K2E-EVM].
> >>
> >>What happens there is that without this patch SCSI layer sets a wrong
> >>bounce buffer limit in scsi_calculate_bounce_limit() for the USB mass
> >>storage device. dma_max_pfn() evaluates to 0x8fffff and bounce_limit
> >>is set to 0x8fffff000 whereas maximum DMA'ble physical memory on Keystone 2
> >>is 0x87fffffff. This results in non DMA'ble pages being given to the
> >>USB controller and hence the lock-up.
> >>
> >>NOTE: in the above case, USB-SCSI-device's dma_pfn_offset was showing as 0.
> >>This should have really been 0x780000 as on K2e, LOWMEM_START is 0x80000000
> >>and HIGHMEM_START is 0x800000000. DMA zone is 2GB so dma_max_pfn should be
> >>0x87ffff. The incorrect dma_pfn_offset for the USB storage device is because
> >>USB devices are not correctly inheriting the dma_pfn_offset from the
> >>USB host controller. This will be fixed by a separate patch.
> >
> >I'd like to hear from Santosh, as the author of the original change.
> >The original commit doesn't mention which platform it was intended for
> >or what the problem was, which would've been helpful.
> >
> From what I recollect, we did these changes to make the max pfn behave
> same on ARM arch as other archs. This patch was evolved as part of
> fixing the max*pfn assumption.

To me, the proposed patch _looks_ correct, because...

> >>diff --git a/arch/arm/include/asm/dma-mapping.h b/arch/arm/include/asm/dma-mapping.h
> >>index d009f79..bf02dbd 100644
> >>--- a/arch/arm/include/asm/dma-mapping.h
> >>+++ b/arch/arm/include/asm/dma-mapping.h
> >>@@ -111,7 +111,7 @@ static inline dma_addr_t virt_to_dma(struct device *dev, void *addr)
> >> /* The ARM override for dma_max_pfn() */
> >> static inline unsigned long dma_max_pfn(struct device *dev)
> >> {
> >>-	return PHYS_PFN_OFFSET + dma_to_pfn(dev, *dev->dma_mask);
> >>+	return dma_to_pfn(dev, *dev->dma_mask);
> >> }
> >> #define dma_max_pfn(dev) dma_max_pfn(dev)
> By doing this change I hope we don't break other drivers on Keystone so
> am not sure about the change.

dma_to_pfn() returns the page frame number referenced from physical
address zero - the default implementation of dma_to_pfn() is
bus_to_pfn(), which is __phys_to_pfn(x), which is just x >> PAGE_SHIFT.
The other thing about dma_to_pfn() is that it should return a
zero-referenced PFN number, where PFN 0 = physical address 0.

If there is some offset for keystone2, that should be taken care of
via "dev->dma_pfn_offset", and not offsetting the return value from
dma_to_pfn().

So I'm 99.9% convinced that the proposed change is correct.

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