Re: EHCI PCI bug on PPC7448 platforms.

On Wed, 2007-04-25 at 08:48 -0700, Dale Farnsworth wrote:
> sjhill wrote:
> > I have hit a bug using a PPC7448 processor with a PCI EHCI USB
> > controller. Atomic operations for reference counting are failing
> > and I am interested to hear what others have to say on this.
> > 
> > Looking in the MPC7450 RISC Microprocessor Family Reference Manual
> > (MPC7450UM, Rev. 5, 1/2005):
> > 
> >    http://www.freescale.com/files/32bit/doc/ref_manual/MPC7450UM.pdf
> > 
> > On Page 102 in Table 1-2, the DSI 0x300 exception can be caused by:
> > 
> >    "A lwarx or stwcx. instruction to memory with cache-inhibited
> >     or write-through memory/cache access atrributes."
> > 
> > For the functions qh_get/gh_put in drivers/usb/host/ehci-mem.c,
> > the kref_get/kref_put functions are called which perform atomic
> > operations using the PPC atomic assembly instructions.
> > 
> > The problem is that all of the ehci_qh and ehci_qtd blocks are
> > allocated through dma_pool_alloc which marks all of the pages
> > cache-inhibit. Thus, whenever the kref functions are called and
> > the PPC-specific atomic operations are performed, the lwarx and
> > stwcx instructions are executed causing kernel panics because of
> > the DSI 0x300 exception.
> > 
> > Currently, I comment out the ref-counting and the EHCI controller
> > performs perfectly on my 7448 platform. However, this is certainly
> > not the solution going forward. Is it truly necessary to have the
> > ref count be in DMA-able memory? I know it is convenient to allocate
> > the entire structure all at once, but this certainly will not work
> > for 7448 platforms. Comments? Thanks.
> 
> Good find Steve.  That's nasty and does need to get fixed.

It is really old problem, see the thread.

http://marc.info/?l=linux-usb-devel&m=115921549316531&w=2

-Dave



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