On Mon, May 21, 2018 at 5:16 AM, Vinod Koul <vinod.koul@xxxxxxxxxx> wrote: >> >> I understand the residue can't be read after terminate, that's why >> reading the residue is step 2 in pause/residue/terminate. My question >> was whether the entire sequence pause/residue/terminate taken as a >> whole can or cannot lose data. Saying that individual steps can or >> can't lose data is not enough, context is required. The key point is >> whether pause flushes in-flight data to its destination or not. If it >> does, and our residue is accurate, the terminate cannot cause data >> loss. If pause doesn't flush, an additional step of flush_sync as >> Lars suggested is required. So pause/flush_sync/residue/terminate >> would be the safe sequence that cannot lose data. > > I wouldn't use cannot, shouldn't would be better here as it depends on HW and > where all data has been buffered and if it can be flushed or not. > > Have you checked if pl330 supports such flushing? It does not, at least in the context of pausing. The dma-330 DMAEND instruction flushes in-flight data to its destination, and there are read/write barrier instructions also, but none of them can be injected on the fly into a running dma thread. DMAKILL can be, but it discards in-flight data. Currently, if an 8250 serial driver uses the pl330 for rx dma, the result is possible data loss/corruption. If there was a stronger pause capability, call it "cmd_sync_pause" which guaranteed flushing of in-flight data to its destination and accurate residue reading when paused, then the 8250 serial driver could check for "cmd_sync_pause" and reject dma drivers that do not have that capability. pl330.c would not advertise cmd_sync_pause. I don't know if other dmaengine hardware would be able to support cmd_sync_pause or not, I'm mostly just familiar with the pl330. The ep93xx_dma.c driver for example has a m2p_hw_synchronize function which seems to do a flush. -- Frank -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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