Hi Franklin, On Wed, 13 Apr 2016 15:08:12 -0500 "Franklin S Cooper Jr." <fcooper@xxxxxx> wrote: > > > On 03/21/2016 10:04 AM, Boris Brezillon wrote: > > Hi Franklin, > > > > On Thu, 10 Mar 2016 17:56:42 -0600 > > Franklin S Cooper Jr <fcooper@xxxxxx> wrote: > > > >> Based on DMA documentation and testing using high memory buffer when > >> doing dma transfers can lead to various issues including kernel > >> panics. > > > > I guess it all comes from the vmalloced buffer case, which are not > > guaranteed to be physically contiguous (one of the DMA requirement, > > unless you have an iommu). > > > >> > >> To workaround this simply use cpu copy. The amount of high memory > >> buffers used are very uncommon so no noticeable performance hit should > >> be seen. > > > > Hm, that's not necessarily true. UBI and UBIFS allocate their buffers > > using vmalloc (vmalloced buffers fall in the high_memory region), and > > those are likely to be dis-contiguous if you have NANDs with pages > 4k. > > > > I recently posted patches to ease sg_table creation from any kind of > > virtual address [1][2]. Can you try them and let me know if it fixes > > your problem? > > It looks like you won't be going forward with your patchset based on > this thread [1]. Nope. According to Russell it's unsafe to do that. > I can probably reword the patch description to avoid > implying that it is uncommon to run into high mem buffers. Also DMA with > NAND prefetch suffers from a reduction of performance compared to CPU > polling with prefetch. This is largely due to the significant over head > required to read such a small amount of data at a time. The > optimizations I've worked on all revolved around reducing the cycles > spent before executing the DMA request. Trying to make a high memory > buffer able to be used by the DMA adds significant amount of cycles and > your better off just using the cpu for performance reasons. Okay. One comment though, why not using virt_addr_valid() instead of addr >= high_memory here? Best Regards, Boris -- Boris Brezillon, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
