Messages in this thread | | | Subject | Re: [PATCH 12/16] staging/comedi: mark as broken | From | Ian Abbott <> | Date | Mon, 17 Jun 2019 14:15:43 +0100 |
| |
On 14/06/2019 16:34, Christoph Hellwig wrote: > On Fri, Jun 14, 2019 at 05:30:32PM +0200, Greg KH wrote: >> On Fri, Jun 14, 2019 at 04:48:57PM +0200, Christoph Hellwig wrote: >>> On Fri, Jun 14, 2019 at 04:02:39PM +0200, Greg KH wrote: >>>> Perhaps a hint as to how we can fix this up? This is the first time >>>> I've heard of the comedi code not handling dma properly. >>> >>> It can be fixed by: >>> >>> a) never calling virt_to_page (or vmalloc_to_page for that matter) >>> on dma allocation >>> b) never remapping dma allocation with conflicting cache modes >>> (no remapping should be doable after a) anyway). >> >> Ok, fair enough, have any pointers of drivers/core code that does this >> correctly? I can put it on my todo list, but might take a week or so... > > Just about everyone else. They just need to remove the vmap and > either do one large allocation, or live with the fact that they need > helpers to access multiple array elements instead of one net vmap, > which most of the users already seem to do anyway, with just a few > using the vmap (which might explain why we didn't see blowups yet).
Avoiding the vmap in comedi should be do-able as it already has other means to get at the buffer pages.
When comedi makes the buffer from DMA coherent memory, it currently allocates it as a series of page-sized chunks. That cannot be mmap'ed in one go with dma_mmap_coherent(), so I see the following solutions.
1. Change the buffer allocation to allocate a single chunk of DMA coherent memory and use dma_mmap_coherent() to mmap it.
2. Call dma_mmap_coherent() in a loop, adjusting vma->vm_start and vma->vm_end for each iteration (vma->vm_pgoff will be 0), and restoring the vma->vm_start and vma->vm_end at the end.
I'm not sure if 2 is a legal option.
-- -=( Ian Abbott <abbotti@mev.co.uk> || Web: www.mev.co.uk )=- -=( MEV Ltd. is a company registered in England & Wales. )=- -=( Registered number: 02862268. Registered address: )=- -=( 15 West Park Road, Bramhall, STOCKPORT, SK7 3JZ, UK. )=-
| |