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

[Santosh Shilimkar]

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.

commit 26ba47b18318abe7dadbe9294a611c0e932651d8
Author: Santosh Shilimkar <santosh.shilimkar@xxxxxx>
Date:   Thu Aug 1 03:12:01 2013 +0100

    ARM: 7805/1: mm: change max*pfn to include the physical offset of 
memory

    Most of the kernel code assumes that max*pfn is maximum pfns because
    the physical start of memory is expected to be PFN0. Since this
    assumption is not true on ARM architectures, the meaning of max*pfn
    is number of memory pages. This is done to keep drivers happy which
    are making use of of these variable to calculate the dma bounce limit
    using dma_mask.

    Now since we have a architecture override possibility for DMAable
    maximum pfns, lets make meaning of max*pfns as maximum pnfs on ARM
    as well.

> > Fixes: 6ce0d2001692 ("ARM: dma: Use dma_pfn_offset for dma address translation")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > Cc: Olof Johansson <olof@xxxxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
> > Reported-by: Grygorii Strashko <grygorii.strashko@xxxxxx>
> > Signed-off-by: Roger Quadros <rogerq@xxxxxx>
> > ---
> >  arch/arm/include/asm/dma-mapping.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > 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.