On Sun, 28 Aug 2011, David Gibson wrote: > I've never been very clear on what exactly DMA resources cover, DMA resource data are usually DMA request line ID numbers. DMA request lines are dedicated, unidirectional hardware signals from I/O devices to one or more independent DMA controllers[1][2][3]. Request lines indicate when the device is ready to accept data from the DMA controller, or, when the device has data ready for the DMA controller to fetch. On a given DMA controller, each request line has its own unique ID number[4][5][6]. During a DMA transfer, the I/O device asserts or deasserts its DMA request lines as its FIFOs fill or drain[7][8]. This will pause or resume the DMA transfer, if the DMA controller is programmed with the correct DMA request line IDs[9][10][11][12]. In a hardware-synchronized DMA transfer, the DMA controller must be controlled by the device because it is not good to read from an empty device FIFO, or to write to a full device FIFO. There aren't many sane courses of action in those circumstances with most interconnects and DMA controllers. So in terms of Linux kernel code, the driver needs the DMA request line ID numbers so it and the DMA code can program the DMA controller to synchronize DMA transfers with the I/O device(s). Like hardware IRQ IDs, these DMA request line IDs are properties of a DMA controller itself, but are associated with individual I/O devices. And like IRQ IDs, these DMA request line ID mappings may change from chip version to chip version[13], even for the same IP block. So it is useful for device data formats to provide DMA request line IDs dynamically. Several upstream device drivers get their DMA request line IDs from the device data format[14][15][16]. But more drivers should be doing this than currently are[17]: - the device driver author may have hardcoded the DMA request line ID, assuming it would never change - DMA could be broken on the device due to hardware bugs, so it is unused - the driver author may just never have gotten around to implementing DMA, or was reassigned to another project, or couldn't figure it out - the device may have its own internal DMA controller logic, so support for an external DMA controller was simply never added When looking at a driver, it may not be obvious which of the cases apply. (N.B., I/O device IP blocks that contain their own dedicated DMA controllers that are specialized to transfer only to memory don't need DMA request lines.) - Paul All code citations below are of Linux v3.1-rc3. 1. A DMA controller's view of DMA request lines. Figure 9-4, "SDMA Controller Integration". _OMAP34xx Multimedia Device Silicon Revision 3.1.x Version R (SWPU223R)_ (public version). http://focus.ti.com/pdfs/wtbu/OMAP34xx_ES3.1.x_PUBLIC_TRM_vZR.zip 2. A simple device's view of DMA request lines, only connected to one DMA controller. Figure 22-17, "MMC/SD/SDIO1 Integration". _OMAP34xx Multimedia Device Silicon Revision 3.1.x Version R (SWPU223R)_ (public version). http://focus.ti.com/pdfs/wtbu/OMAP34xx_ES3.1.x_PUBLIC_TRM_vZR.zip 3. A device's view of its DMA request lines connected to two separate DMA controllers, and thus with distinct numbering. Note also that the device refers to its signals with strings, while the DMA controllers refer to the same signals with ID numbers. Figure 21-16, "McBSP1 Integration". _OMAP34xx Multimedia Device Silicon Revision 3.1.x Version R (SWPU223R)_ (public version). http://focus.ti.com/pdfs/wtbu/OMAP34xx_ES3.1.x_PUBLIC_TRM_vZR.zip 4. Table 5-1 "Peripheral Addresses and Selectors". _RMI Alchemy Au1550 Security Network Processor Data Book Revision E_. May 2007. Available from http://www.poeticmonkey.com/ebay/semiconductors/cpus/au1550/au1550_db_0507e.pdf 5. Table 6-7 "Priority and Default Mapping of Peripheral to DMA". _ADSP-BF51x Blackfin Processor Hardware Reference Preliminary Revision 0.1_ (January 2009). Available from http://www.analog.com/static/imported-files/processor_manuals/bf51x_hwr_rev_0-1.pdf 6. Table 16-5 "sDMA Controller Request Mapping". _OMAP4460 Multimedia Device Silicon Revision 1.x Version H (Public Version) (SWPU235H)_. Available from http://focus.ti.com/pdfs/wtbu/OMAP4460_ES1.x_PUBLIC_TRM_vH.zip 7. A really nice illustration of the interaction between a FIFO and DMA request line during DMA receive. Figure 17-25 "Receive FIFO DMA Request Generation (32 Characters)". _OMAP34xx Multimedia Device Silicon Revision 3.1.x Version R (SWPU223R)_ (public version). http://focus.ti.com/pdfs/wtbu/OMAP34xx_ES3.1.x_PUBLIC_TRM_vZR.zip 8. A similarly good illustration of the interaction between a FIFO and DMA request line during DMA transmit. Figure 17-26 "Transmit FIFO DMA Request Generation (56 Spaces)". _OMAP34xx Multimedia Device Silicon Revision 3.1.x Version R (SWPU223R)_ (public version). http://focus.ti.com/pdfs/wtbu/OMAP34xx_ES3.1.x_PUBLIC_TRM_vZR.zip 9. Table 3-23 "DMACCxConfiguration Register bit assignments". _PrimeCell DMA Controller (PL080) Technical Reference Manual (DDI0196G)_. http://infocenter.arm.com/help/topic/com.arm.doc.ddi0196g/DDI0196.pdf 10. Section 16.4.9.2 "Hardware Synchronization". _OMAP4460 Multimedia Device Silicon Revision 1.x Version H (Public Version) (SWPU235H)_. Available from http://focus.ti.com/pdfs/wtbu/OMAP4460_ES1.x_PUBLIC_TRM_vH.zip 11. Page 6-5 "Peripheral DMA". _ADSP-BF51x Blackfin Processor Hardware Reference Preliminary Revision 0.1_ (January 2009). Available from http://www.analog.com/static/imported-files/processor_manuals/bf51x_hwr_rev_0-1.pdf 12. Section 5.3.1.1 "Command 0". _RMI Alchemy Au1550 Security Network Processor Data Book Revision E_. May 2007. Available from http://www.poeticmonkey.com/ebay/semiconductors/cpus/au1550/au1550_db_0507e.pdf 13. For example, consider what happens to the EXT_DMAREQ3 request line ID on the SDMA controller: ... from the OMAP2420: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/arm/plat-omap/include/plat/dma.h;h=dc562a5c0a8ad2c7117b0b02e43b85cabe7b9aa5;hb=c11a7e26f8ee60bda0e64983291113ce5d04df55#l113 ... to the OMAP2430: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/arm/plat-omap/include/plat/dma.h;h=dc562a5c0a8ad2c7117b0b02e43b85cabe7b9aa5;hb=c11a7e26f8ee60bda0e64983291113ce5d04df55#l125 ... to the OMAP3430: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/arm/plat-omap/include/plat/dma.h;h=dc562a5c0a8ad2c7117b0b02e43b85cabe7b9aa5;hb=c11a7e26f8ee60bda0e64983291113ce5d04df55#l179 14. An OMAP2+ driver example is: drivers/tty/serial/omap-serial.c lines 1249 and 1255: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=drivers/tty/serial/omap-serial.c#l1249 15. An Alchemy AU1xxx example is: sound/soc/au1x/dbdma2.c lines 173 and 177: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=sound/soc/au1x/dbdma2.c#l173 16. A Blackfin BF51x example is: drivers/spi/spi-bfin5xx.c line 1323: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=drivers/spi/spi-bfin5xx.c#l1323 17. First Law of Kernel Hacking: the bodies are always buried in the device driver code. -- 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
