Re: [PATCH v2] media: videobuf2-dc: set properly dma_max_segment_size

[Lars-Peter Clausen <lars@metafoo.de>]

On 08/13/2015 03:49 PM, Marek Szyprowski wrote:
> Hello,
> 
> On 2015-08-13 14:40, Lars-Peter Clausen wrote:
>> On 08/12/2015 11:58 AM, Marek Szyprowski wrote:
>>> If device has no DMA max_seg_size set, we assume that there is no limit
>>> and it is safe to force it to use DMA_BIT_MASK(32) as max_seg_size to
>>> let DMA-mapping API always create contiguous mappings in DMA address
>>> space. This is essential for all devices, which use dma-contig
>>> videobuf2 memory allocator.
>>>
>>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
>>> ---
>>> Changelog:
>>> v2:
>>> - set max segment size only if a new dma params structure has been
>>>    allocated, as suggested by Laurent Pinchart
>>> ---
>>>   drivers/media/v4l2-core/videobuf2-dma-contig.c | 15 +++++++++++++++
>>>   1 file changed, 15 insertions(+)
>>>
>>> diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c
>>> b/drivers/media/v4l2-core/videobuf2-dma-contig.c
>>> index 94c1e64..455e925 100644
>>> --- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
>>> +++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
>>> @@ -862,6 +862,21 @@ EXPORT_SYMBOL_GPL(vb2_dma_contig_memops);
>>>   void *vb2_dma_contig_init_ctx(struct device *dev)
>>>   {
>>>       struct vb2_dc_conf *conf;
>>> +    int err;
>>> +
>>> +    /*
>>> +     * if device has no max_seg_size set, we assume that there is no limit
>>> +     * and force it to DMA_BIT_MASK(32) to always use contiguous mappings
>>> +     * in DMA address space
>>> +     */
>>> +    if (!dev->dma_parms) {
>>> +        dev->dma_parms = kzalloc(sizeof(*dev->dma_parms), GFP_KERNEL);
>>> +        if (!dev->dma_parms)
>>> +            return ERR_PTR(-ENOMEM);
>>> +        err = dma_set_max_seg_size(dev, DMA_BIT_MASK(32));
>>> +        if (err)
>>> +            return ERR_PTR(err);
>>> +    }
>> I'm not sure if this is such a good idea. The DMA provider is responsible
>> for setting this up. We shouldn't be overwriting this here on the DMA
>> consumer side. This will just mask the bug that the provider didn't setup
>> this correctly and might cause bugs on its own if it is not correct. It will
>> lead to conflicts with DMA providers that have multiple consumers (e.g.
>> shared DMA core). And also the current assumption is that if a driver
>> doesn't set this up explicitly the maximum segement size is 65536.
> 
> The problem is that there is no good place for changing this extremely low
> default
> value. V4L2 media devices, which use videobuf2-dc expects to get buffers mapped
> contiguous in the DMA/IO address space. Initially I wanted to have a code for
> setting dma max segment size directly in the dma-mapping subsystem. This
> however
> causeed problems in the other places, as mentioned in the following mail:
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/305913.html

And the same reasoning in that reply still applies here. Try to fix the DMA
provider drivers to setup the correct value.

- Lars