[microblaze-uclinux] Device drivers for a Linux kernel with caches but no MMU support

[Paul Hartke <phartke@xxxxxxxxxxxx>]

I'm trying to wrap my mind around writing device drivers for a Linux kernel
with caches but no MMU support.  Although Microblaze currently only offers
write-through caches, coherency with a DMA engine must be enforced by
invalidating any cache lines that the DMA engine could end up touching. 
This coherency issue is generally bypassed by allocating DMA buffer
descriptors in non-cachable memory:

drivers/net/xilinx_enet/adapter.c
        /* calc size of descriptor space pool; alloc from non-cached memory
*/
        dftsize = (XEM_DFT_RECV_DESC + XEM_DFT_SEND_DESC) *
            sizeof (XBufDescriptor);

        lp->desc_space = consistent_alloc(GFP_ATOMIC, dftsize,
                                          &lp->desc_space_handle);
        if (lp->desc_space == 0) {
                return -1;
        }
        lp->desc_space_size = dftsize;

However, how does Microblaze uclinux do this exactly without an MMU?  One
solution is to have Microblaze itself have a chunk of noncached memory and
put the descriptors there.  But, how would the kernel know about this
memory?  In addition, most microblaze designs I've seen cache the whole
memory space.

The other approach is to invalidate the desciptors every time they are
passed to the DMA engine but, at least for the case of the Xilinx Ethernet
drivers, means mucking with them.

How are folks handling this?

Thanks.
Paul

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