Hi Robin!
On 22 January 2015 at 00:26, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> Hi Sumit,
>
>
> On 21/01/15 04:16, Sumit Semwal wrote:
>>
>> From: Rob Clark <robdclark@xxxxxxxxx>
>>
>> For devices which have constraints about maximum number of segments in
>> an sglist. For example, a device which could only deal with contiguous
>> buffers would set max_segment_count to 1.
>>
>> The initial motivation is for devices sharing buffers via dma-buf,
>> to allow the buffer exporter to know the constraints of other
>> devices which have attached to the buffer. The dma_mask and fields
>> in 'struct device_dma_parameters' tell the exporter everything else
>> that is needed, except whether the importer has constraints about
>> maximum number of segments.
>>
>> Signed-off-by: Rob Clark <robdclark@xxxxxxxxx>
>> [sumits: Minor updates wrt comments on the first version]
>> Signed-off-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
>> ---
>> include/linux/device.h | 1 +
>> include/linux/dma-mapping.h | 19 +++++++++++++++++++
>> 2 files changed, 20 insertions(+)
>>
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index fb50673..a32f9b6 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -647,6 +647,7 @@ struct device_dma_parameters {
>> * sg limitations.
>> */
>> unsigned int max_segment_size;
>> + unsigned int max_segment_count; /* INT_MAX for unlimited */
>> unsigned long segment_boundary_mask;
>> };
>>
>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> index c3007cb..38e2835 100644
>> --- a/include/linux/dma-mapping.h
>> +++ b/include/linux/dma-mapping.h
>> @@ -154,6 +154,25 @@ static inline unsigned int
>> dma_set_max_seg_size(struct device *dev,
>> return -EIO;
>> }
>>
>> +#define DMA_SEGMENTS_MAX_SEG_COUNT ((unsigned int) INT_MAX)
>> +
>> +static inline unsigned int dma_get_max_seg_count(struct device *dev)
>> +{
>> + return dev->dma_parms ?
>> + dev->dma_parms->max_segment_count :
>> + DMA_SEGMENTS_MAX_SEG_COUNT;
>> +}
>
>
> I know this copies the style of the existing code, but unfortunately it also
> copies the subtle brokenness. Plenty of drivers seem to set up a dma_parms
> struct just for max_segment_size, thus chances are you'll come across a
> max_segment_count of 0 sooner or later. How badly is that going to break
> things? I posted a fix recently[1] having hit this problem with
> segment_boundary_mask in IOMMU code.
>
Thanks very much for reviewing this code; and apologies for missing
your patch that you mentioned here; sure, I will update my patch
accordingly as well.
>> +
>> +static inline int dma_set_max_seg_count(struct device *dev,
>> + unsigned int count)
>> +{
>> + if (dev->dma_parms) {
>> + dev->dma_parms->max_segment_count = count;
>> + return 0;
>> + } else
>
>
> This "else" is just as unnecessary as the other two I've taken out ;)
>
>
> Robin.
>
> [1]:http://article.gmane.org/gmane.linux.kernel.iommu/8175/
>
>
>> + return -EIO;
>> +}
>> +
>> static inline unsigned long dma_get_seg_boundary(struct device *dev)
>> {
>> return dev->dma_parms ?
>>
>
>
BR,
Sumit.
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