* Alan Stern <[EMAIL PROTECTED]> [070328 12:10]:
> On Wed, 28 Mar 2007, Tony Lindgren wrote:
>
> > Hi,
> >
> > Here's a little patch to make sure tranfer_buffer is always if DMA
> > is temporarily unavailable. I ran into this with tusb6010 that
> > currently can only do limited DMA. This fixes a problem of connecting
> > storage devices and doing mixed DMA/PIO access.
>
> Your patch isn't correct, or at least, it is very misleading. It calls
> page_address() in a situation where it cannot be certain that the page
> frame lies in accessible kernel memory. On some systems the value it
> calculates will be garbage.
>
> At a minimum you need to add a comment explaining that if the page isn't
> accessible, the transfer_buffer value won't ever be used.
Yeah it's a bit ugly. I don't think there's currently an easy way
to optimize out mapping of transfer_buffer on such systems.
Here's a version with better comments.
Tony
From: Tony Lindgren <[EMAIL PROTECTED]>
Subject: USB: Always map transfer_buffer
In case a DMA channel for an endpoint is temporarily
unavailable, the host controller driver may still be
able to transfer the data using PIO. Without this patch
the transfer_buffer is not mapped, and host controller
does not know what to use for PIO.
Signed-off-by: Tony Lindgren <[EMAIL PROTECTED]>
Index: linux-omap-2.6/drivers/usb/core/message.c
===================================================================
--- linux-omap-2.6.orig/drivers/usb/core/message.c 2007-03-28 10:59:21.000000000 -0400
+++ linux-omap-2.6/drivers/usb/core/message.c 2007-03-28 14:35:00.000000000 -0400
@@ -412,16 +412,19 @@ int usb_sg_init (
io->urbs [i]->status = -EINPROGRESS;
io->urbs [i]->actual_length = 0;
+ /*
+ * Some systems need both transfer_buffer and transfer_dma
+ * to revert to PIO if DMA is temporarily unavailable.
+ * Note that transfer_buffer may not be accessible on some
+ * systems. In that case there is no need to use it either.
+ */
+ io->urbs [i]->transfer_buffer =
+ page_address (sg [i].page) + sg [i].offset;
if (dma) {
- /* hc may use _only_ transfer_dma */
io->urbs [i]->transfer_dma = sg_dma_address (sg + i);
len = sg_dma_len (sg + i);
- } else {
- /* hc may use _only_ transfer_buffer */
- io->urbs [i]->transfer_buffer =
- page_address (sg [i].page) + sg [i].offset;
+ } else
len = sg [i].length;
- }
if (length) {
len = min_t (unsigned, len, length);
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