> On Tue, Jan 17, 2006 at 11:40:53AM +0200, Riisisulg wrote:
>
> > Hello. I'm using AMD Alchemy db1550 with au1550, it has 2.6.15
kernel
> > patched with linux-2.6.14.5-mips-1.patch, i have compiled cross
compiler
> > for the platform (but not checked it for correctness)
> >
> > since db1550 does not have USB2.0 but instead has two PCI slots, i
> > installed NEC's PCI to USB 2.0 host controller board to it. I was
able
> > to load ehci-hcd driver with success but after the usb mass storage
> > device was inserted the system hanged.
> > After debugging a while i found out that function atomic_add did
not
> > return. I run few tests where atomic add asm code was alerted and
got
> > confidence that sc instruction did not never succeed so the cycle
was
> > repeating forever.
>
> The operation of ll/sc is undefined on uncached memory; non-coherent
> MIPS systems - that is most embedded MIPS systems - will return non
> cache-coherent memory for dma_alloc_coherent. Yes, the naming sucks,
> dma_alloc_{non}coherent do the opposite of what their name is.
>
> Ralf
Thank you for quick answer so i will make some conclusions here
The problem come out by using ehci-hcd, that diver uses it's own dma
pool where it allocates structures of type ehci_qh.
struct ehci_qh has field of kref of type struct kref
in ehci-mem.c file there are functions qh_get(struct ehci_qh *qh),
qh_put(struct ehci_qh *qh) that use functions kref_get and kref_put.
Since ehci_qh structures are allocated from dma pool and both kref_get
and kref_put are using atomic functions that use ll sc then on
non-coherent MIPS system both qh_get and qh_put will hang the system.
The simplest workaround is to replace kref_get and kref_put in
ehci-mem.c or not to use dma pool. I have not cheked other kernel
drivers that use atomic functions on uncached memory.
END, i hope.
