On Fri, May 29, 2020 at 12:26:10PM +0300, Andy Shevchenko wrote: > On Fri, May 29, 2020 at 11:12:04AM +0300, Serge Semin wrote: > > On Fri, May 29, 2020 at 10:55:32AM +0300, Andy Shevchenko wrote: > > > On Fri, May 29, 2020 at 7:02 AM Serge Semin > > > <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > Even if DMA transactions are finished it doesn't mean that the SPI > > > > transfers are also completed. It's specifically concerns the Tx-only > > > > SPI transfers, since there might be data left in the SPI Tx FIFO after > > > > the DMA engine notifies that the Tx DMA procedure is done. In order to > > > > completely fix the problem first the driver has to wait for the DMA > > > > transaction completion, then for the corresponding SPI operations to be > > > > finished. In this commit we implement the former part of the solution. > > > > > > > > Note we can't just move the SPI operations wait procedure to the DMA > > > > completion callbacks, since these callbacks might be executed in the > > > > tasklet context (and they will be in case of the DW DMA). In case of > > > > slow SPI bus it can cause significant system performance drop. > > > > > > > > I read commit message, I read the code. What's going on here since you > > > repeated xfer_completion (and its wait routine) from SPI core and I'm > > > wondering what happened to it? Why we are not calling > > > spi_finalize_current_transfer()? > > > > We discussed that in v4. You complained about using ndelay() for slow SPI bus, > > which may cause too long atomic context execution. We agreed. Since we can't wait > > in the tasklet context and using a dedicated kernel thread for waiting would be too > > much, Me and Mark agreed, that > > > even if it causes us of the local wait-function > > re-implementation the best approach would be not to use the generic > > spi_transfer_wait() method, but instead wait for the DMA transactions locally > > in the DMA driver and just return 0 from the transfer_one callback indicating > > that the SPI transfer is finished and there is no need for SPI core to wait. As > > a lot of DMA-based SPI drivers do. > > The above is missed in the commit message. > > > If you don't understand what the commit message says, just say so. I'll > > reformulate it. > > See above. A bit of elaboration would be good. Thank you! Agreed. I'll create a more detailed commit description, which will have the info you cited. -Sergey > > -- > With Best Regards, > Andy Shevchenko > >
