On Wed, Jan 19, 2011 at 02:16:13PM -0800, Guenter Roeck wrote: > On Wed, 2011-01-19 at 16:14 -0500, Ben Dooks wrote: > > On Wed, Jan 19, 2011 at 01:26:37PM -0800, Guenter Roeck wrote: > > > Signed-off-by: Guenter Roeck <[email protected]> > [ ... ] > > > > +/* Structure to hold all of our device specific stuff */ > > > +struct i2c_diolan_u2c { > > > + struct usb_device *usb_dev; /* the usb device for this device */ > > > + struct usb_interface *interface;/* the interface for this device */ > > > + struct i2c_adapter adapter; /* i2c related things */ > > > + int olen; /* Output buffer length */ > > > + int ocount; /* Number of enqueued messages */ > > > + u8 obuffer[DIOLAN_OUTBUF_LEN]; /* output buffer */ > > > + u8 ibuffer[DIOLAN_INBUF_LEN]; /* input buffer */ > > > > you should cache-line align the buffers to avoid any problems with > > dma-vs-cache, etc. > > > There are several other drivers which don't cache-align the buffers used > with usb_bulk_msg(). Do those all have potential problems, or is this > only relevant for optimization ? > > If it is just for optimization, I'd argue that the speed here is so slow > that it won't make much of a difference, if any, if the buffers were > aligned.
It is more to do with the way some architectures/cpus deal with cache coherency with resepect to DMA... in some cases it is possible for hardware to be dealing with a buffer that the cpu may still be using if they are not appropriately aligned. I forget the exact case, but losing a few bytes in a buffer is preferable to the other case. I also believe that X86 style architectures do not have this problem. -- Ben Dooks, [email protected], http://www.fluff.org/ben/ Large Hadron Colada: A large Pina Colada that makes the universe disappear. -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
