Date: Thu, 23 Aug 2012 17:45:03 -0600 From: Ian Lepore <freebsd@damnhippie.dyndns.org> To: Warner Losh <imp@bsdimp.com> Cc: freebsd-arm@freebsd.org, freebsd-mips@freebsd.org, freebsd-arch@freebsd.org Subject: Re: Partial cacheline flush problems on ARM and MIPS Message-ID: <1345765503.27688.602.camel@revolution.hippie.lan> In-Reply-To: <FD8DC82C-AD3B-4EBC-A625-62A37B9ECBF1@bsdimp.com> References: <1345757300.27688.535.camel@revolution.hippie.lan> <3A08EB08-2BBF-4B0F-97F2-A3264754C4B7@bsdimp.com> <1345763393.27688.578.camel@revolution.hippie.lan> <FD8DC82C-AD3B-4EBC-A625-62A37B9ECBF1@bsdimp.com>
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On Thu, 2012-08-23 at 17:26 -0600, Warner Losh wrote: > > On Aug 23, 2012, at 3:28 PM, Ian Lepore wrote: > > Now we have a new type of constraint, I think of it as "granularity". > > In effect, we have a DMA system that can only do DMA in cacheline sized > > chunks. Even when the IO size -- and thus the number of "bits on the > > wire" -- is less than the cacheline size, at the end of the DMA > > operation (which includes the software-assisted coherency operations) > > the number of bytes in memory that may be modified is the size of a > > cacheline. This is because "the DMA system" is not just the engine that > > moves bytes around, it's the combination of hardware and software that > > work together to maintain cache coherency. > But this isn't new. It is an alignment requirement, which carries > with it an implicit size requirement. If you enforce the alignment, > and force all 'sub buffers' to have this alignment, you don't need the > new thing. So do you think it's safe to assume that any given dma tag that has an alignment constraint also implicitly has a buffer size constraint that the size must be a multiple of the alignment? What if we have a platform with a 32-byte cacheline / DMA granularity, and then we have a builtin device on that SoC which can only do DMA on a 64K alignment (which its tag would reflect), but the hardware can move as little as 1 byte at a time? Children of that bridge device come along and allocate little 16-byte buffers that eat 16 pages each. It doesn't seem all that far-fetched to me. -- Ian
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