> -----Original Message----- > From: David Brownell [mailto:davi...@pacbell.net] > Sent: Thursday, June 11, 2009 1:09 PM > To: Tivy, Robert > Cc: davinci-linux-open-source@linux.davincidsp.com; Andrea Gasparini > Subject: Re: EDMA with user provided virtual addresses. > > On Thursday 11 June 2009, Tivy, Robert wrote: > > EDMA requires physically contiguous buffers, > > so in addition to needing to know the physical address, > > you also must ensure that your buffers are contiguous physically, > > Not really. The usual way to handle that sort of thing is to > make whatever user-to-kernel mapping call you need, which > returns something along the lines of a scatterlist. > Then just construct a chain of DMA transfers, one for each segment. > > So a 12 Kbyte userspace buffer might scan three pages: > > page N+0 offset 1K, len 3K > page N+1 offset 0, len 4K > page N+2 offset 0, len 1K > > Map that to the kernel, and it'll still be three pages but > the page addresses will differ ... say, pages X, Y, and Z. > Offsets and lengths will be unchanged. Now just feed EDMA a > chain of three transfers. > > That kind of logic is easy to wrap in utility functions. > (If it were wrapped, would non-DSP code really need CMEM?) >
Good point, thanks for letting me know. So, the requirement for phys contig buffers is mostly for HW accelerators that address memory w/o the benefit of an MMU or without chaining capability. Having one contig buf for EDMA also makes things simpler to handle, but isn't necessary. CMEM can help in the non-DSP case for those HW accelerators. One last advantage of CMEM - ARM/DSP-based combo code can move over to ARM only without any porting, or using a different allocator. - Rob_______________________________________________ Davinci-linux-open-source mailing list Davinci-linux-open-source@linux.davincidsp.com http://linux.davincidsp.com/mailman/listinfo/davinci-linux-open-source
