On Wed, Dec 17, 2008 at 10:37 PM, Kanigeri, Hari <h-kanigeri2@xxxxxx> wrote: > Hi Felipe, > >> I received information from TI saying that user-space buffers >> need to be 128-byte aligned for the DSP to work properly. >> >> Is this true or is it some kind of limitation on their MMU >> code that might be solved by Hiroshi's iommu? >> >> This impacts quite drastically user-space applications like >> GStreamer since a memory copy would be required to achieve >> that 128-byte alignment. >> >> They also claim that adding 128-byte padding at the beginning >> and at the end achieves the same purpose. > > This is to address the memory corruption that might arise when using DMM (Dynamic memory mapping) feature. > When the DSP processes data, the DSP cache controller loads 128-Byte chunks (lines) from SDRAM and writes the data back in 128-Byte chunks. If a DMM buffer does not start and end on a 128-Byte boundary, the data preceding the start address from the 128-Byte boundary to the Start Address and the data at addresses trailing the end address from the End Address to the next 128-Byte boundary will be loaded and written back as well. If the DMM buffer was allocated on the heap of a process running on the ARM, the preceding and trailing data, if modified by the ARM (or other non-DSP entity) during DSP processing, will be overwritten by the DSP cache controller, when the 128-Byte chunks are written back to SDRAM. This can lead to heap corruption. Thanks for the explanation. So it doesn't have anything to do with IOMMU; it's the software running on the DSP that has this limitation. > I think if you request the buffer from OMX component, this issue will be taken as there is an additional 256 bytes padding. Yes, but in that case a memcpy would be unavoidable in many cases, like trying to render the result of video decoding on Xv. If the Xv buffer is unaligned; a dedicated DMM would need to be allocated, and then the data coming from DSP would need to be copied to the Xv buffer. That has a huge performance penalty. So if I'm understanding correctly there's no problem with unaligned DMM buffers if the DSP software is just reading; the problem is when writing data back to ARM. -- Felipe Contreras -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
