Re: [PATCH 04/12] v4l: vb2-dma-contig: add setup of sglist for MMAP buffers

[Subash Patel <subashrp@gmail.com>]

Hi Laurent, Tomasz,

On 06/10/2012 11:28 PM, Laurent Pinchart wrote:
> Hi Tomasz,
>
> On Friday 08 June 2012 16:31:31 Tomasz Stanislawski wrote:
>> Hi Laurent and Subash,
>>
>> I confirm the issue found by Subash. The function vb2_dc_kaddr_to_pages does
>> fail for some occasions. The failures are rather strange like 'got 95 of
>> 150 pages'. It took me some time to find the reason of the problem.
>>
>> I found that dma_alloc_coherent for iommu an ARM does use ioremap_page_range
>> to map a buffer to the kernel space. The mapping is done by updating the
>> page-table.
>>
>> The problem is that any process has a different first-level page-table. The
>> ioremap_page_range updates only the table for init process. The PT present
>> in current->mm shares a majority of entries of 1st-level PT at kernel range
>> (above 0xc0000000) but *not all*. That is why vb2_dc_kaddr_to_pages worked
>> for small buffers and occasionally failed for larger buffers.
>>
>> I found two ways to fix this problem.
>> a) use&init_mm instead of current->mm while creating an artificial vma
>> b) access the dma memory by calling
>>     *((volatile int *)kaddr) = 0;
>>     before calling follow_pfn
>>     This way a fault is generated and the PT is
>>     updated by copying entries from init_mm.
>>
>> What do you think about presented solutions?
>
> Just to be sure, this is a hack until dma_get_sgtable is available, and it
> won't make it to mainline, right ?  In that case using init_mm seem easier.
Although I agree adding a hack for timebeing, why not use the 
dma_get_sgtable() RFC itself to solve this in clean way? The hacks 
anyways cannot go into mainline when vb2 patches get merged.

Regards,
Subash
>