On 2.4 on 32bit hosts it has no direct effect because the driver interface has not
been revised. It's only useful on 64bit atm.
2.5 will hopefully move the bounce test into your driver. Currently it tests
PageHighMem() which is bogus (and gives the 1GB boundary), because it should test
the physical address against the dma_mask.
On 64bit PageHighMem always returns false, so the bounce buffer is never create in
the upper level and the final decision is down in your driver.
Currently create_bounce() in the block layer is at a very unfortunate place on the
top half of ll_rw_blk, because there it has not even a chance to know about the
final device (e.g. when LVM or MD come into play)
I suspect someone will quickly do a limited 2.4 patch that moves create_bounce a bit
down for selected devices on 32bit hosts -- it is a very obvious optimization.
It probably also could be done with minor changes.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to firstname.lastname@example.org
Please read the FAQ at http://www.tux.org/lkml/