[PATCH 2/8] ARM: Implement read/write for ownership in theARMv6 DMA cache ops

[George G. Davis]

Hi,

On Thu, May 13, 2010 at 03:15:20PM +0100, Catalin Marinas wrote:
> On Thu, 2010-05-13 at 14:54 +0100, George G. Davis wrote:
> > Hi,
> > 
> > On Thu, May 13, 2010 at 09:26:57AM +0100, Catalin Marinas wrote:
> > > On Thu, 2010-05-13 at 06:27 +0100, Ronen Shitrit wrote:
> > > > Our ARM v6 does have speculative prefetch...
> > >
> > > That's the I-cache and we should be ok with it. What may affect this
> > > patch is speculative fetches into the D-cache.
> > 
> > prefetch()?
> 
> Drivers should not use prefetch (PLD) on memory handed over to a device
> for DMA transfers. To me this is like explicitly accessing such memory
> during a DMA transfer.

I agree and was merely speculating that prefetch() may be a case where
D-Cache lines are explicitly prefetched and may be the cause of this
problem, e.g. from e1000_clean_rx_irq() in drivers/net/e1000e/netdev.c:

	while (rx_desc->status & E1000_RXD_STAT_DD) {
		struct sk_buff *skb;
		u8 status;

		if (*work_done >= work_to_do)
			break;
		(*work_done)++;

		status = rx_desc->status;
		skb = buffer_info->skb;
		buffer_info->skb = NULL;

		prefetch(skb->data - NET_IP_ALIGN);

		i++;
		if (i == rx_ring->count)
			i = 0;
		next_rxd = E1000_RX_DESC(*rx_ring, i);
		prefetch(next_rxd);

		next_buffer = &rx_ring->buffer_info[i];

		cleaned = 1;
		cleaned_count++;
		pci_unmap_single(pdev,
				 buffer_info->dma,
				 adapter->rx_buffer_len,
				 PCI_DMA_FROMDEVICE);


I'm to lazy^Wbusy to drill down but could the above prefetch(skb->data...)
before pci_unmap_single() or similar cases be causing the problem reported
by Ronen?

I think the above may be one of those "if drivers are expecting data in
the cache to be preserved over a dma_unmap_area() call, they are buggy
and need fixing." Russell noted in a previous post, no?

--
Regards,
George