On Fri, Sep 20, 2024 at 05:56:23PM +0300, Serge Semin wrote: > On Fri, Sep 20, 2024 at 05:24:37PM +0300, Andy Shevchenko wrote: > > On Fri, Sep 20, 2024 at 12:33:51PM +0300, Serge Semin wrote: > > > On Mon, Sep 16, 2024 at 04:01:08PM +0300, Andy Shevchenko wrote: ... > > > There is another problem caused by the too slow coherent memory IO on > > > my device. Due to that the data gets to be copied too slow in the > > > __dma_rx_complete()->tty_insert_flip_string() call. As a result a fast > > > incoming traffic overflows the DW UART inbound FIFO. But that can be > > > worked around by decreasing the Rx DMA-buffer size. (There are some > > > more generic fixes possible, but they haven't shown to be as effective > > > as the buffer size reduction.) > > > This sounds like a specific quirk for a specific platform. In case you > > are going to address that make sure it does not come to be generic. > > Of course reducing the buffer size is the platform-specific quirk. > > A more generic fix could be to convert the DMA-buffer to being > allocated from the DMA-noncoherent memory _if_ the DMA performed by > the DW DMA-device is non-coherent anyway. In that case the > DMA-coherent memory buffer is normally allocated from the > non-cacheable memory pool, access to which is very-very slow even on > the Intel/AMD devices. So using the cacheable buffer for DMA, then > manually invalidating the cache for it before DMA IOs and prefetching > the data afterwards seemed as a more universal solution. But my tests > showed that such approach doesn't fully solve the problem on my > device. That said that approach permitted to execute data-safe UART > transfers for up to 460Kbit/s, meanwhile just reducing the buffer from > 16K to 512b - for up to 2.0Mbaud/s. It's still not enough since the > device is capable to work on the speed 3Mbit/s, but it's better than > 460Kbaud/s. Ah, interesting issue. Good lick with solving it the best way you can. Any yes, you're right that 2M support is better than 0.5M. -- With Best Regards, Andy Shevchenko
