On Thu, Jan 25, 2018 at 03:47:54PM +0100, Maxime Chevallier wrote: > The issue boils down to this (for the sake of clarity, I simplified the > function by taking the following assumptions : > - We work on a vmalloced buffer > - max_seg_size < PAGE_SIZE > - len > max_seg_size (len is size of the buffer we want to split) ... > min = min_t(size_t, > len, desc_len - offset_in_page(buf)); > The 'min' variable computation seems incorrect when max_seg_size is less > than PAGE_SIZE, in particular the "desc_len - offset_in_page(buf)", > which should be something like "(len % PAGE_SIZE) - offset_in_page(buf)" If I'm understanding you correctly the issue you're identifying here is that we ignore max_seg_size here and instead just use PAGE_SIZE (you don't explicitly say what you think is wrong so I might be missing something here). > Unfortunately, this start to be complex if we want to take all possible > cases into account : If we want to split a buffer in segments, where > some might cross page boundaries, how do we know how much sg elements > will there be ? > I thought about using something like __sg_alloc_table_from_pages, but > the "max_segment" parameter is expected to be page aligned, which is not > the case here. Perhaps I'm missing something here but surely we just need to handle the case where desc_len is bigger than offset_in_page()? The number of scatterlist entries we're going to need is at most DIV_ROUND_UP(PAGE_SIZE / max_seg_size) * DIV_ROUND_UP(len / PAGE_SIZE) (ie, the number of segments we need per page multiplied by the number of pages covered by the data) which it looks like we already have mostly handled. > I found this issue with a prototype implementation of SPI DMA xfers on > armada 3700, using fpga-pgr as a spi device. In my case, max_seg_size is > set to 512 bytes. Like I say I might be missing something here. > This should occur with other DMA / SPI controllers, as long as the > max_seg_size is less than PAGE_SIZE. I'm not sure how likely that is TBH - 512 is an extremely low limit for a DMA controller.
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