Re: [PATCH 12/16] staging/comedi: mark as broken

Ian Abbott
2019-06-17T13:15:43Z

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.

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