Re: [PATCH] xhci: use dma_alloc_coherent with fully cleared pages

[Mathias Nyman]

On 19.02.2015 02:34, Tim Chen wrote:
> 
> Commit d92ef66c4f8f ("x86: make dma_alloc_coherent() return zeroed memory
> if CMA is enabled") changed the dma_alloc_coherent page clearance from
> using an __GFP_ZERO in page allocation to not setting the flag but doing
> an explicit memory clear at the end.
> 
> However the memory clear only covered the memory size that
> was requested, but may not be up to the full extent of the
> last page, if the total pages returned exceed the
> memory size requested.  This behavior has caused problem with XHCI
> and caused it to hang:
> 
> kernel: xhci_hcd 0000:00:14.0: Stopped the command ring failed, maybe the host is dead
> kernel: xhci_hcd 0000:00:14.0: Abort command ring failed
> kernel: xhci_hcd 0000:00:14.0: HC died; cleaning up
> kernel: xhci_hcd 0000:00:14.0: Error while assigning device slot ID
> kernel: xhci_hcd 0000:00:14.0: Max number of devices this xHCI host supports is 64.
> 
> This patch reverts xhci the use the old behavior to avoid the hang.  XHCI should
> do a proper fix to avoid this un-allocated memory.
> 
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.16+
> ---
>  drivers/usb/host/xhci-mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> index 5cb3d7a..39e7196 100644
> --- a/drivers/usb/host/xhci-mem.c
> +++ b/drivers/usb/host/xhci-mem.c
> @@ -1658,7 +1658,7 @@ static int scratchpad_alloc(struct xhci_hcd *xhci, gfp_t flags)
>  		goto fail_sp;
>  
>  	xhci->scratchpad->sp_array = dma_alloc_coherent(dev,
> -				     num_sp * sizeof(u64),
> +				     PAGE_ALIGN(num_sp * sizeof(u64)),
>  				     &xhci->scratchpad->sp_dma, flags);
>  	if (!xhci->scratchpad->sp_array)
>  		goto fail_sp2;
> 

Thanks for figuring out that the issue was in allocation of scratchpad memory.

Turns out we don't calculate the num_sp correctly in the first place, we didn't include
the additional high order field of the max scratchpad buffers xhci tells we should reserve.
I got a patch for this and will send it to forward to Greg once 3.20-rc1 is tagged

-Mathias
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