Re: [PATCH v5] OMAP UART: Add omap-serial driver support.

[Tony Lindgren <tony@atomide.com>]

* Govindraj <govindraj.ti@gmail.com> [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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