On Tue, 13 Sep 2016, Arnd Bergmann wrote: > On Tuesday, September 13, 2016 11:16:03 AM CEST Roger Quadros wrote: > > If dma_pfn_offset is not inherited correctly from the host controller, > > it might result in sub-optimal configuration as bounce > > buffer limit might be set to less than optimal level. > > > > Consider the mass storage device case. > > USB storage driver creates a scsi host for the mass storage interface in > > drivers/usb/storage/usb.c > > The scsi host parent device is nothing but the the USB interface device. > > Now, __scsi_init_queue() calls scsi_calculate_bounce_limit() to find out > > and set the block layer bounce limit. > > scsi_calculate_bounce_limit() uses dma_max_pfn(host_dev) to get the > > bounce_limit. host_dev is nothing but the device representing the > > mass storage interface. > > If that device doesn't have the right dma_pfn_offset, then dma_max_pfn() > > is messed up and the bounce buffer limit is wrong. > > > > e.g. On Keystone 2 systems, dma_max_pfn() is 0x87FFFF and dma_mask_pfn > > is 0xFFFFF. Consider a mass storage use case: Without this patch, > > usb scsi host device (usb-storage) will get a dma_pfn_offset of 0 resulting > > in a dma_max_pfn() of 0xFFFFF within the scsi layer > > (scsi_calculate_bounce_limit()). > > This will result in bounce buffers being unnecessarily used. > > > > Hint: On 32-bit ARM platforms dma_max_pfn() = dma_mask_pfn + dma_pfn_offset > > > > Signed-off-by: Roger Quadros <rogerq@xxxxxx> > > Acked-by: Arnd Bergmann <arnd@xxxxxxxx> Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
