On 08-05-18, 10:36, Frank Mori Hess wrote: > On Tue, May 8, 2018 at 5:04 AM, Marek Szyprowski > <m.szyprowski@xxxxxxxxxxx> wrote: > > Hi Frank and Vinod, > > > > On 2018-04-28 23:50, Frank Mori Hess wrote: > >> This reverts commit 88987d2c7534a0269f567fb101e6d71a08f0f01d. > >> > >> The pl330.c pause implementation violates the dmaengine requirement > >> for no data loss, since it relies on the DMAKILL > >> instruction. However, DMAKILL discards in-flight data from the > >> dma controller's fifo. This is documented in the dma-330 manual > >> and I have observed it with hardware doing device-to-memory burst > >> transfers. The discarded data may or may not show up in the > >> residue count, depending on timing (resulting in data corruption > >> effectively). > >> > >> Signed-off-by: Frank Mori Hess <fmh6jj@xxxxxxxxx> > > > > This revert completely breaks serial driver operation on almost all Exynos > > SoCs, because serial driver relies on having PAUSE feature and proper > > residue reporting from dma engine. Please drop it if possible. Hi Marek, I would appreciate if you can review the pl330 changes as that clearly seems to impact you. This was in review for quite a bit > It will cause the serial driver to not use the pl330.c driver for dma, > the serial driver will fall back on using the cpu. This is > unfortunate, but the dma hardware simply does not support pause. The > "nice" stop instruction DMAEND is not allowed to be inserted using the > debug instruction register. The only possibility for implementing > pause would be to make the dma transfer do a DMAWFE (wait for event) > before every transfer. Then you would need to devote another dma > thread to doing nothing but DMASEV (send event) to keep the transfer > going. The pause could then DMAKILL the event-generating thread > rather than the transfer thread. I don't know exactly what the > performance impact would be, but it couldn't be good. > > The serial driver could be modified to still use dma for TX, since it > only needs pause for RX. Also, if your serial hardware can report > exactly how many bytes it has sitting in its rx fifo, the serial > driver could be modified to use pause-less dma for RX. This is > actually what I did for the custom serial hardware I'm using with a > dma-330, although our serial hardware has a very large rx fifo which > makes this scheme worthwhile. That makes sense to me. If dma doesnt support, then why should SW claim broken support.. -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
