Hi, I would like to mmap a kernel buffer, allocated with pci_alloc_consistent() for DMA, to userspace and came up with the following. Since there seem to be some (unresolved) issues (see below) with this and I would like to do the RightThing(TM), I would appreciate your comments about my stuff.
As for the unresolved issues, I found the following LKML threads to be very helpful in understanding the problem: 1. (DMA API issues) http://marc.theaimsgroup.com/?l=linux-kernel&m=108757847518687&w=2 2. (can device drivers return non-ram via vm_ops->nopage?) http://marc.theaimsgroup.com/?l=linux-kernel&m=107978968703503&w=2 What I did (with comments on problematic / not fully clear to me parts): pci_probe(): my_buffer = pci_alloc_consistent() (size: PAGE_SIZE << 4) ------------------------------------------------------------------------ my_mmap(): vma->vm_ops = &my_vm_ops; vma->vm_flags |= (VM_RESERVED | VM_IO); my_vma_open(vma); my_vm_ops = { .open = my_vm_open, .close = my_vm_close, .nopage = my_vm_nopage, } Q: Is VM_IO needed here? I took it from the sg.c driver. Q: I choosed nopage because remap_page_range does not work on RAM pages. Correct? ------------------------------------------------------------------------ my_vm_open(): increment vma usage count my_vm_close(): decrement vma usage count my_vm_nopage(): offset = (address - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT); pageptr = my_buffer + offset; page = virt_to_page(pageptr); /* got it, now increment the count */ get_page(page); if (type) { *type = VM_FAULT_MINOR; } return page; Q: As seen in the threads mentioned above I should not use virt_to_page() on addresses I got from pci_alloc_consistent/dma_alloc_coherent. What is the right way to handle this? Q: get_page() increments the page refcount. Where is the correspondig put_page() operation? -------------------------------------------------------------------------- pci_remove(): pci_free_consistent(my_buffer, ...) -------------------------------------------------------------------------- I first tried the above and failed. Somehow my driver seemed to screw up the page tables. I noticed a function sg_correct4mmap() in the sg.c driver which "fixes" refcount handling on pages allocated using __get_free_pages() with order > 0. After implementing this things improved. Here are the changes: my_mmap(): if (!mmap_called) { correct4mmap(my_buffer, 1); mmap_called = 1; } release(): if (mmap_called) { if (vma_usage_count == 0) { correct4mmap(my_buffer, 0) mmap_called = 0; } } Q: Can someone please briefly explain, why (if at all) this is needed? ----------------------------------------------------------------------------- Somehow the whole thing does not "feel" right so I would really like some comments on this. I think this could be helpful to other, too. Quoting Jeff Garzik from one of the older threads: "My suggestion/request to the VM wizards would be to directly provide mmap helpers for dma/mmio/pio, that Does The Right Thing. And require their use in every driver. Don't give driver writers the opportunity to think about this stuff and/or screw it up." There are such helper functions on ARM and Russel tried to push them into the generic DMA API (the last time in June 2004, I think). Can any progress be expected regarding these helper functions? Thanks for your time. -Rolf -- Rolf Offermanns <[EMAIL PROTECTED]> SYSGO AG Tel.: +49-6136-9948-0 Am Pfaffenstein 14 Fax: +49-6136-9948-10 55270 Klein-Winternheim http://www.sysgo.com - 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/
