* Govindraj <govindraj.ti@xxxxxxxxx> [100210 06:36]:
> Tony,
>
> >
> >> +static void serial_omap_start_tx(struct uart_port *port)
> >> +{
> >> + struct uart_omap_port *up = (struct uart_omap_port *)port;
> >> +
> >> + if (up->use_dma && !(up->port.x_char)) {
> >> +
> >> + struct circ_buf *xmit = &up->port.state->xmit;
> >> + unsigned int start = up->uart_dma.tx_buf_dma_phys +
> >> + (xmit->tail & (UART_XMIT_SIZE - 1));
> >> + if (uart_circ_empty(xmit) || up->uart_dma.tx_dma_state)
> >> + return;
> >> + spin_lock(&(up->uart_dma.tx_lock));
> >> + up->uart_dma.tx_dma_state = 1;
> >> + spin_unlock(&(up->uart_dma.tx_lock));
> >> +
> >> + up->uart_dma.tx_buf_size = uart_circ_chars_pending(xmit);
> >> + /*
> >> + * It is a circular buffer. See if the buffer has wounded back.
> >> + * If yes it will have to be transferred in two separate dma
> >> + * transfers
> >> + */
> >> + if (start + up->uart_dma.tx_buf_size >=
> >> + up->uart_dma.tx_buf_dma_phys + UART_XMIT_SIZE)
> >> + up->uart_dma.tx_buf_size =
> >> + (up->uart_dma.tx_buf_dma_phys +
> >> + UART_XMIT_SIZE) - start;
> >> +
> >> + if (up->uart_dma.tx_dma_channel == 0xFF)
> >> + omap_request_dma(up->uart_dma.uart_dma_tx,
> >> + "UART Tx DMA",
> >> + (void *)uart_tx_dma_callback, up,
> >> + &(up->uart_dma.tx_dma_channel));
> >
> > You need to check the result from omap_request_dma. On a busy system
> > it's quite possible that all DMA channels are taken and you need to
> > dynamically fall back to PIO transfer instead.
> >
> > Looks like this function is easy to fix for that. Maybe you also
> > need to reprogram something in the UART for switching between DMA
> > and PIO?
>
> I explored the possibility of falling back to to Interrupt mode when we get an
> -EBUSY signal from request dma.
>
> It is quite complex, as we set DMA mode in FCR [Fifo control register]
> and FCR can be configured only when "the baud clock is not running
> (DLL_REG and DLH_REG set to 0)."
>
> So we need to reconfigure complete register set since setting DLL and
> DLH to zero means
> we need to reconfigure for baudrate again this would affect the mcr
> configuration and other
> flow control configuration which is set. Thus increasing the
> complexity of switching
> to interrupt mode.
Sure, that's what I figured too.. How about make the reconfigure function
non __init for that?
Sounds like it will also be needed if we want to power off the uart
for off-idle too.
This is quite a critical driver to get right, it's the only debug
console we have in most cases.
Regards,
Tony
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