On Thu, Aug 26, 2010 at 07:00:24PM +0900, FUJITA Tomonori wrote: > On Thu, 26 Aug 2010 11:53:11 +0200 > Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > > > > We have currently a number of boards broken in the mainline. They must be > > > > fixed for 2.6.36. I don't think the mentioned API will do this for us. So, > > > > as I suggested earlier, we need either this or my patch series > > > > > > > > http://thread.gmane.org/gmane.linux.ports.sh.devel/8595 > > > > > > > > for 2.6.36. > > > > > > Why can't you revert a commit that causes the regression? > > > > > > The related DMA API wasn't changed in 2.6.36-rc1. The DMA API is not > > > responsible for the regression. And the patchset even exnteds the > > > definition of the DMA API (dma_declare_coherent_memory). Such change > > > shouldn't applied after rc1. I think that DMA-API.txt says that > > > dma_declare_coherent_memory() handles coherent memory for a particular > > > device. It's not for the API that reserves coherent memory that can be > > > used for any device for a single device. > > The patch that made the problem obvious for ARM is > > 309caa9cc6ff39d261264ec4ff10e29489afc8f8 aka v2.6.36-rc1~591^2~2^4~12. > > So this went in before v2.6.36-rc1. One of the "architectures which > > similar restrictions" is x86 BTW. > > > > And no, we won't revert 309caa9cc6ff39d261264ec4ff10e29489afc8f8 as it > > addresses a hardware restriction. > > How these drivers were able to work without hitting the hardware restriction? Well, OMAP processors have experienced lock-ups due to multiple mappings of memory, so the restriction in the architecture manual is for real. But more the issue is that the behaviour you get from a region is _totally_ unpredictable (as the arch manual says). With the VIPT caches, they can be searched irrespective of whether the page tabkes indicate that it's supposed to be cached or not - which means you can still hit cache lines for an ioremap'd region. And if you do, how do you know that the cached data is still valid - what if it's some critical data that results in corruption - how do you know whether that's happened or not? It might not even cause a kernel exception. We have to adhere to the restrictions placed upon us by the architecture at hand, and if that means device drivers break, so be it - at least we get to know what needs to be fixed for these restrictions. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
