Hi Clemens,
On Sun, Sep 3, 2017 at 9:24 PM, Clemens Gruber
<clemens.gruber@xxxxxxxxxxxx> wrote:
> Hi,
>
> I finished my analysis of the imx RS-485 DMA problem, we discussed on
> the list before and which was fixed upstream with commit 514ab34dbad6
> ("serial: imx: Prevent TX buffer PIO write when a DMA has been started")
> from Ian.
>
> Open questions were:
> 1) Why did this only appear on SMP machines?
> 2) Why did this only affect RS-485?
>
> Thanks to ftrace, I can now answer both:
>
> 1) When starting a new transmission, the following happens in
> imx_start_tx:
> After imx_port_rts_active to enable the external RS-485 transceiver,
> the transmit-complete-interrupt is enabled (TCEN).
> Then, imx_dma_tx is called and the DMA request is handed off.
> Now, while sdma_prep_slave_sg and sdma_load_context are executing on one
> CPU core, imx_int is called on another, because the previously enabled
> interrupt asserted immediately (TXDC was active because both the TxFIFO
> and the Shifter were empty) after enabling it. (50us on i.MX6Q SMP)
> This leads to the transmit_buffer function (called from imx_int/imx_txint)
> writing to UTXD which corrupts the ongoing DMA transmission and also leads to
> xmit->tail getting double-incremented and jumping over xmit->head, so the whole
> UART_XMIT_SIZE xmit buffer is sent out.
> On non-SMP machines, the interrupt is handled later when
> uart_circ_empty(xmit) is already true and transmit_buffer returns before
> doing PIO. With a non-SMP kernel on an i.MX6Q, I measured 35ms until
> imx_int/imx_txint and the function_graph trace showed
> imx_txint()
> imx_stop_tx()
> (Instead of a trace_printk I added to the PIO loop in transmit_buffer)
>
> 2) The transmit-complete-interrupt (TCEN) is only used in RS-485 mode.
Thanks for sharing your detailed analysis!
Glad to have this issue fixed in mainline now.
Thanks
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