> -----Original Message----- > From: Russell King - ARM Linux [mailto:linux@xxxxxxxxxxxxxxxx] > Sent: Tuesday, July 27, 2010 9:37 PM > To: Shilimkar, Santosh > Cc: Mankad, Maulik Ojas; linux-usb@xxxxxxxxxxxxxxx; linux- > omap@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > Subject: Re: Issue with file transfers to a mass storage device on SMP > system > > On Tue, Jul 27, 2010 at 07:59:17PM +0530, Shilimkar, Santosh wrote: > > Once it's pushed out of L2X WB, it will hit the memory. Just to give an > additional data point, with CATC analyzer what we see that only those > > 16 bytes CDB are 0x0. This buffer was memset to 0x0 just before the > > memcpy. > > > > I am just wondering who will issue a barrier(wmb) on this buffer before > DMA > > start if we don't use dma-mapping APIs? May be for dma_alloc_coherent > > buffers, we need to explicitly issue the barrier. > > wmb's don't take addresses - they're a global thing. All stores before > the wmb() take effect before stores after the wmb(). > > The wmb() is issued by Catalin's IO ordering patches: > > +#define writeb(v,c) ({ wmb(); writeb_relaxed(v,c); }) > +#define writew(v,c) ({ wmb(); writew_relaxed(v,c); }) > +#define writel(v,c) ({ wmb(); writel_relaxed(v,c); }) > > So, the wmb() is issued to ensure that all stores to (eg) buffers > allocated by dma_alloc_coherent() hit memory prior to the store to > the device. > > Now, if you're writing to registers using something other than > write[bwl](), > you'll miss the wmb(), and therefore your DMA buffer won't be up to date. > Yep. > And this _is_ the problem: > > drivers/usb/musb/musb_io.h:static inline void musb_writew(void __iomem > *addr, unsigned offset, u16 data) > drivers/usb/musb/musb_io.h: { __raw_writew(data, addr + offset); } > drivers/usb/musb/musb_io.h:static inline void musb_writel(void __iomem > *addr, unsigned offset, u32 data) > drivers/usb/musb/musb_io.h: { __raw_writel(data, addr + offset); } > drivers/usb/musb/musb_io.h:static inline void musb_writeb(void __iomem > *addr, unsigned offset, u8 data) > drivers/usb/musb/musb_io.h: __raw_writew(tmp, addr + (offset & ~1)); > drivers/usb/musb/musb_io.h:static inline void musb_writeb(void __iomem > *addr, unsigned offset, u8 data) > drivers/usb/musb/musb_io.h: { __raw_writeb(data, addr + offset); } > > All IO performed by musb misses out on the barriers - so what you > need to do is either add wmb()s to these, or you need to ensure > that the driver has the various necessary memory barriers in place. > The latter solution will be more efficient. I also agree that patching driver is less impacting and non-intrusive. We will go ahead with the original work-around which adds barrier in the driver. Thanks for the good discussion on this. Regards, Santosh -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
