On Thu, 2005-07-14 at 16:39, Andrew Morton wrote: > Daniel McNeil <[EMAIL PROTECTED]> wrote: > > > > Do drivers have problems with odd addresses or with > > non-512 addresses? > > I do recall hearing rumours that some bus-masters have fairly strict memory > alignment requirements. A cacheline size, perhaps - that would be 32 bytes > given the age of the hardware. > > But yeah, it's v. risky to assume that all bus masters can cope with > memory alignments down to two bytes. > > It would be sane to put the minimum alignment into ->backing_dev_info, > default to 512, get the device drivers to override that as they are tested. > > But this introduces a very very bad problem: people will write applications > which work on their hardware, ship the things and then find that the apps > break on other people's hardware. So we can't do that. > > Instead, we need to work out the minimum alignment requirement for all disk > controllers and DMA controllers and motherboards in the world. And that > includes catering for weird ones which appear to work but which > occasionally fail in mysterious ways with finer alignments. That's hard. > It's easier to continue to make application developers jump through hoops.
I was hoping this patch would help turn rumors into real data :) If we did put min alignment into backing_dev_info, we could implement the equivalent of bounce buffers for direct-io -- or just fall back to buffer i/o like it does sometimes anyway. That way application would not break, just get worse performance on some hardware. Right now I just wanted to get the issues on table, get some test results, and see how to proceed from there. Since this patch only affects direct i/o, getting test results shouldn't cause too many problems. Thanks, Daniel - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
