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Subjectinteresting effect of the get_user_pages and copy_to_user
Dear Linux kernel team

I am working on PCIe device driver and need to transfer up to 20MB data,
unfortunately the device has no scatter/gather controller, so I have to do in usual way.

in the code it done in the following way (shortlty):

long ioctl_dma(struct file *filp, unsigned int *cmd_p, unsigned long *arg_p){
unsigned long arg;
arg = *arg_p;
#define SG_MAX_ORDER 10
max_order_length = (2<<(SG_MAX_ORDER-1))*PAGE_SIZE;
ppWriteBuf = (void *)__get_free_pages(GFP_KERNEL , SG_MAX_ORDER);
pTmpDmaHandle = pci_map_single(pdev, pWriteBuf, max_order_length, PCI_DMA_FROMDEVICE);

1. copy_from_user(&dma_data, (device_ioctrl_dma*)arg, (size_t)io_dma_size)) // get user buffer and data
2. tmp_dma_size = dma_data.dma_size; // get DMA size from user buffer
3. nr_entries = tmp_dma_size/max_order_length; // how many DMAs has to be done
4. for(int i=0; i < nr_entries; ++i){
Make DMA;
pci_dma_sync_single_for_cpu(pdev, pTmpDmaHandle, max_order_length, PCI_DMA_FROMDEVICE);
copy_to_user ((void *)(arg + tmp_user_offset), pWriteBuf, max_order_length)
pci_dma_sync_single_for_device(pdev,pTmpDmaHandle, max_order_length, PCI_DMA_FROMDEVICE);
tmp_user_offset += max_order_length;
this work fine and gives in user application around 87ms for 20MB.
Than I just do:
get_user_pages( (unsigned long)arg, // start
pDmaUnit->nr_pages, // length in pages
1, // >0 --> write to user space
0, // force. drivers should set 0

The DMA time goes to ~50ms (was 87ms)



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