On Thu, Sep 19, 2019 at 01:40:03PM +0200, Philipp Puschmann wrote: > Hi Uwe > > Am 19.09.19 um 13:22 schrieb Uwe Kleine-König: > > On Thu, Sep 19, 2019 at 12:26:28PM +0200, Philipp Puschmann wrote: > >> Using only 4 DMA periods for UART RX is very few if we have a high > >> frequency of small transfers - like in our case using Bluetooth with > >> many small packets via UART - causing many dma transfers but in each > >> only filling a fraction of a single buffer. Such a case may lead to > >> the situation that DMA RX transfer is triggered but no free buffer is > >> available. While we have addressed the dma handling already with > >> "dmaengine: imx-sdma: fix dma freezes" we still want to avoid > > > > Is this statement still true now that you split this patch out of your > > bigger series? > Yes. The dma patches care about stopping DMA channel. This patch tries to > avoid that the channel runs out of usable buffers (aka dma periods). > > > > >> UART RX FIFO overrun. So we decrease the size of the buffers and > >> increase their number and the total buffer size. > > > > What happens when such an RX FIFO overrun happens? Are characters lost? > > Or only time? > Good question. In explanation i have missed an important point: > When using HW flowcontrol via RTS/CTS and the buffer is full CTS is used to > tell the remote device - here the Bluetooth chip - to stop sending data. > For a while this prevents losing of characters. But then the remote device > comes into trouble as its internal TX buffers runs over. Depends on the > device how it handles this case and if it recovers if data flow is enabled > again. > > In case without HW flow control characters would be lost. Depends on the upper > layer what happens then. > > > Does your change have an influence if I do fewer but > > bigger transfers? > Don't think so. The dma periods are raw data buffers. If one is full the next one > is being used. For the performance i don't see a significant difference between > using 1 kB buffers or 4 kB buffers. Would be great to add these infos to the commit log. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |
