Re: [linux-usb-devel] USB on PPC440GP (cache incoherent)

Roland Dreier wrote:

I had to make some changes to the USB driver to get this working as
there are still some places where structures on the stack are being
used for DMA.

Good to have that -- there have periodically been passes made through
drivers to fix such problems, evidently at least 2.4 didn't catch all
of them.  Corresponding changes should get into 2.5 kernels too.

Note that this might not work perfectly on all cache-incoherent
processors, since kmalloc could potentially allocate a chunk of memory
that is smaller than the processor's cache line size.  However it is
safe on the 440GP since the 440GP's cache line size is 32 bytes.

Could you elaborate?  Documentation/DMA-mapping.txt says that kmalloc
returns data suitable for DMA, you are saying otherwise.  The DMA
mapping calls are supposed to handle cache flushing as needed.  If
they don't, a lot of code will be breaking ...

 struct usb_hub {
 	struct usb_device *dev;

 	struct urb *urb;		/* Interrupt polling pipe */

-	char buffer[(USB_MAXCHILDREN + 1 + 7) / 8]; /* add 1 bit for hub status change */
-					/* and add 7 bits to round up to byte boundary */
+        char *buffer;
 	int error;
 	int nerrors;

This hub.h change (and its follow-ons) should not be necessary since the
struct usb_hub is already allocated using kmalloc() in hub_probe(), and
so it's DMA-ready.  (Modulo the spec issue noted above for DMA-mapping.txt
of course!)

- Dave


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