Date:	Wed, 19 Nov 2008 16:39:23 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Hommel, Thomas (GE EntSol, Intelligent Platforms)" 
	<Thomas.Hommel@...anuc.com>, USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: ISP1760 driver crashes

On Wed, Nov 19 2008, Alan Stern wrote:
> On Wed, 19 Nov 2008, Jens Axboe wrote:
> 
> > On Wed, Nov 19 2008, Alan Stern wrote:
> > > On Wed, 19 Nov 2008, Sebastian Andrzej Siewior wrote:
> > > 
> > > > Hommel, Thomas (GE EntSol, Intelligent Platforms) wrote:
> > > > 
> > > > > I indeed have HIGHMEM enabled in my configuration.
> > > > > I recompiled the kernel without HIGHMEM and it works. I don't think that
> > > > > this a satisfying solution for a board with up to 2GB of RAM and
> > > > > considerable amount of VMALLOC space, but at least this works.
> > > > > If you have any more ideas how to circumvent this, please let me know.
> > > > Sure, this is not a sollution but atleast now I know what happens:
> > > > - The kernel allocates memory for transfer
> > > > - the memory is highmem and not in kernel so the buffer is NULL
> > > > - we don't have a dma-mask and therefore the dma address is 0
> > > > - boom
> > > > 
> > > > The sollution would be probably to prevent the usb-storage core to 
> > > > allocate memory from HIGHMEM.
> > > 
> > > usb-storage doesn't allocate the memory.  The memory is allocated by 
> > > the block layer or the filesystem.
> > > 
> > > > Now I don't if there is a flag for something 
> > > > like that and I am not using that. On the other hand this may be broken 
> > > > for a long time and you are the first one which has that much memory with 
> > > > no DMA-capable USB controller.
> > > 
> > > Jens, is there any way to tell the kernel that a device uses PIO and 
> > > therefore its buffers shouldn't be allocated in high memory?  For 
> > > example, shouldn't a NULL dma_mask do this?
> > 
> > Sure, just use blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH), then you are
> > certain that you will always have a virtual mapping for the IO you
> > receive.
> > 
> > Or you can use the bio kmap/kunmap helpers to get such a mapping
> > temporarily if you wish. But if your pio condition is permanent, you may
> > as well just use bouncing.
> 
> Thank you.
> 
> Thomas, the blk_queue_bounce_limit() routine is called in 
> drivers/scsi/scsi_lib.c:__scsi_alloc_queue().  The value it passes is 
> computed by scsi_calculate_bounce_limit(), and in that routine 
> host_dev->dma_mask should be NULL (since isp1760-hcd sets the mask to 
> NULL).  Therefore the bounce limit should be 0xffffffff.
> 
> Now maybe this value isn't correct.  You can try the patch below to see 
> if it helps.  If it doesn't, add a printk in __scsi_alloc_queue() to 
> see what bounce limit value is getting used.
> 
> Alan Stern
> 
> 
> 
> Index: usb-2.6/drivers/scsi/scsi_lib.c
> ===================================================================
> --- usb-2.6.orig/drivers/scsi/scsi_lib.c
> +++ usb-2.6/drivers/scsi/scsi_lib.c
> @@ -1684,7 +1684,7 @@ static void scsi_request_fn(struct reque
>  u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
>  {
>  	struct device *host_dev;
> -	u64 bounce_limit = 0xffffffff;
> +	u64 bounce_limit = BLK_BOUNCE_HIGH;
>  
>  	if (shost->unchecked_isa_dma)
>  		return BLK_BOUNCE_ISA;
> 

The best solution is probably to either provide a "doesn't do highmem"
in the scsi host template, or provide an appropriate DMA mask for the
pci device to indicate it through that setting instead.

-- 
Jens Axboe

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