-----Original Message-----
From: Oliver Neukum [mailto:oliver at neukum.org]
Sent: Tuesday, February 16, 2010 7:53 PM
To: Shilimkar, Santosh
Cc: Russell King - ARM Linux; Catalin Marinas; Pavel Machek; Greg KH; Matthew Dharm; Sergei Shtylyov;
Ming Lei; Sebastian Siewior; linux-usb at vger.kernel.org; linux-kernel; linux-arm-kernel; Mankad,
Maulik Ojas
Subject: Re: USB mass storage and ARM cache coherency

Am Dienstag, 16. Februar 2010 15:12:45 schrieb Shilimkar, Santosh:

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I am afraid for these controllers the controller driver must be responsible
for all DMA and cache issues. Indicating the exact requirements to the
upper layer would be a battle already lost.
so the safe choice is not to set has_dma and the generic layer will leave
the issue to the lower level.

This means don't use dma at all which will almost kill the performance.

Why would you be unable to map a buffer in the hcd driver when you know
that you'll use DMA?

Probably it can be. The USB stack has the dma maintenance code at common
place for all controllers and hence we were just trying to see if there is
way to handle that way.

This is true. If you can find a clean way to describe your requirements
to the generic layer, that would be better. The problem is that we must
not end up with a dozen flags.

Your original patch however kills ehci, ohci and uhci on some architectures.

How about below approach? Controller driver can set 
"uses_pio_for_control" if it can't do dma for control transfer.

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 80995ef..e3eae02 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c

@@ -1276,7 +1276,7 @@ static int map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
 
 	if (usb_endpoint_xfer_control(&urb->ep->desc)
 	    && !(urb->transfer_flags & URB_NO_SETUP_DMA_MAP)) {
-		if (hcd->self.uses_dma) {
+		if (hcd->self.uses_dma && !hcd->self.uses_pio_for_control) {
 			urb->setup_dma = dma_map_single(
 					hcd->self.controller,
 					urb->setup_packet,

@@ -1335,7 +1335,7 @@ static void unmap_urb_for_dma(struct usb_hcd *hcd, struct urb *urb)
 
 	if (usb_endpoint_xfer_control(&urb->ep->desc)
 	    && !(urb->transfer_flags & URB_NO_SETUP_DMA_MAP)) {
-		if (hcd->self.uses_dma)
+		if (hcd->self.uses_dma && !hcd->self.uses_pio_for_control)
 			dma_unmap_single(hcd->self.controller, urb->setup_dma,
 					sizeof(struct usb_ctrlrequest),
 					DMA_TO_DEVICE);

diff --git a/include/linux/usb.h b/include/linux/usb.h
index d7ace1b..ba5b0a2 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h

@@ -329,6 +329,9 @@ struct usb_bus {
 	int busnum;			/* Bus number (in order of reg) */
 	const char *bus_name;		/* stable id (PCI slot_name etc) */
 	u8 uses_dma;			/* Does the host controller use DMA? */
+	u8 uses_pio_for_control;	/* Does the host controller use PIO
+					 * for control tansfers? 
+					 */
 	u8 otg_port;			/* 0, or number of OTG/HNP port */
 	unsigned is_b_host:1;		/* true during some HNP roleswitches */
 	unsigned b_hnp_enable:1;	/* OTG: did A-Host enable HNP? */

Regards,
Santosh