On Sat, Sep 13, 2008 at 12:15:34PM -0600, Grant Grundler wrote:
> On Fri, Sep 12, 2008 at 11:29:39PM +0200, Felix Frey wrote:
> > Dear readers
> >
> > I have a problem with a linux PCI driver I've written. The driver supports a
> > kind of a data acquisition system, which is implemented in a programable
> > logic device. It's pci interface does not have master capability, so every
> > data exchange is initiated by the CPU.
> >
> > Well, the driver is fairly simple and simply handles data exchange. It has to
> > transfer chunks of data from and to the device. To make it fast I'd like to
> > use burst transfers, which are supported by the device. Within the driver I
> > use the kernel functions memcpy_fromio and memcpy_toio to initiate a
> > transfer.
> > This works so far, however, there are no burst transfers. The bridge does a
> > complete address cycle for every single word which is a big waste of time.
>
> In order to get multiple data cycles per address cycle, either the CPU and/or
> the chipset needs to be told it's ok to coalesce the write transactions
> (memcpy_toio).
Isn't that simply a case of marking the BAR as prefetchable?
Memory locations are characterized as either prefetchable or
non-prefetchable. Prefetchable memory has the following characteristics:
- There are no side effects of a read operation. The read operation
cannot be destructive to either the data or any other state information.
For example, a FIFO that advances to the next data when read would not
be prefetchable. Similarly, a location that cleared a status bit when
read would not be prefetchable.
- When read, the device is required to return all bytes regardless of the
byte enables (four or eight depending upon the width of the data transfer
(refer to Section 3.8.1.)).
- Bridges are permitted to merge writes into this range (refer to
Section 3.2.6.).
--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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