From mboxrd@z Thu Jan 1 00:00:00 1970 From: mkl0301@gmail.com (Lin Mac) Date: Tue, 9 Mar 2010 15:45:54 +0800 Subject: DMA using data buffer vmapped in kernel space In-Reply-To: <20100306193127.GA13262@n2100.arm.linux.org.uk> References: <201003042211.28569.thomas@koeller.dyndns.org> <20100304213634.GH13417@n2100.arm.linux.org.uk> <201003061407.12698.thomas@koeller.dyndns.org> <20100306193127.GA13262@n2100.arm.linux.org.uk> Message-ID: <10d816431003082345j5438d8cdi8ad72c4fa296b684@mail.gmail.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 2010/3/7 Russell King - ARM Linux : > On Sat, Mar 06, 2010 at 02:07:12PM +0100, Thomas Koeller wrote: >> Am Donnerstag, 4. M?rz 2010 22:36:34 schrieb Russell King - ARM Linux: >> > Cache maintainence is done using virtual addresses for L1, and >> > physical addresses for L2. ?There's the need for virtual addresses >> > to be translatable to physical addresses, which is only true for >> > the kernel direct mapped region (pages between PAGE_OFFSET and >> > high_memory). >> >> Isn't the mapping created by vmap() sufficient for the virt/phys >> translation? In which way is this case different from a buffer >> passed in from user space, where the constituent pages are not >> in the directly mapped kernel region either? > > No different. > > The requirement is that dma_map_single() is passed a virtual address > in the kernel direct-mapped memory region, which is translatable using > virt_to_phys() and friends. I had encounter a similiar problem and I simply allocated a new buffer, copy the data, then DMA. It seems slow and stupid. I'm wondering wether could I translate the vmap virt to phys(don't know how to yet), then use phys_to_virt to get the virt in direct-mapped memory region? Is there other possible ways? > Anything which requires a page table lookup to obtain the physical > address is just not acceptable - that requires taking locks and other > messy things, plus is grossly inefficient. Best Regards, Mac Lin