Block stuff is a crappy name, the block layer :-)
> value? Which physical bits it may address, this is a device specific
> attribute and has nothing to with with 4GB and highmem and PCI
> standard specifications. :-)
It didn't make use of this value, it merely provided it for _drivers_ to
use. Driver passes in a max dma address, block layer translates that
into a page address. As for 4GB, see below.
> In fact, this is not only a device specific attribute, it also has
> things to do with elements of the platform.
>
> This is why we have things like pci_dma_supported() and friends.
> Let me give an example, for most PCI controllers on Sparc64 if your
> device can address the upper 2GB of 32-bit PCI space, one may DMA
> to any physical memory location via the IOMMU these controllers have.
>
> There may easily be HIGHMEM platforms which operate this way. So the
> result is that CONFIG_HIGHMEM does _not_ mean ">=4GB memory must be
> bounced".
>
> Really, 0xffffffff is a meaningless value. You have to test against
> device indicated capabilities for bouncing decisions.
Ok, I see where we are not seeing eye to eye. Really, I meant for the
PCI_MAX_DMA32 value to be 'Max address below 4GB' and not 'Max address
we can DMA to with 32-bit PCI'. Does that make sense? Maybe my
explanations weren't quite clear, and of course it didn't really help
that I shoved it in pci.h :-)
> You do not even know how "addressable bits" translates into "range of
> physical memory that may be DMA'd to/from by device". If an IOMMU is
> present on the platform, these two things have no relationship
> whatsoever. These two things happen to have a direct relationship
> on x86, but that is as far as it goes.
That's why I need you to sanity check the cross-platform stuff like
that :-). I see what you mean, point taken. Clearly I need to change the
blk_queue_bounce_limit stuff to check with the PCI capabilities.
> Enough babbling on my part, I'll have a look at your bounce patch
> later today. :-)
Thanks!
-- Jens Axboe- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/