From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oliver Neukum Subject: Re: USB transfer_buffer allocations on 64bit systems Date: Fri, 9 Apr 2010 16:50:31 +0200 Message-ID: <201004091650.31488.oliver@neukum.org> References: Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-usb-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Alan Stern Cc: Daniel Mack , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Pedro Ribeiro , akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, Greg KH , alsa-devel-K7yf7f+aM1XWsZ/bQMPhNw@public.gmane.org, linux-usb-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: alsa-devel@alsa-project.org Am Freitag, 9. April 2010 16:41:48 schrieb Alan Stern: > > It'll work on x86. On incoherent architectures this violates the cacheline > > rules for DMA-mapping if you have to bounce. > > Not true. Consider: The driver allocates a 16-byte buffer (xbuf) > divided up into four sets of four bytes, and sets > > urb[i].transfer_buffer_dma = xbuf_dma + 4*i; > > Then usb_submit_urb(urb[i]) will copy the appropriate four bytes to a > bounce buffer and map the bounce buffer. Accesses to the other parts > of xbuf won't violate the cacheline rules, because xbuf isn't mapped > for DMA -- only the bounce buffer is. When urb[i] completes, the > bounce buffer contents will be copied back to the original four bytes > in xbuf. Again, there is no violation of cacheline rules. I think you are assuming that either every or no part of the buffer is mapped for DMA in place. I don't think you can assume that. Regards Oliver -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html