Known issues with amba-pl010.c driver?
Russell King - ARM Linux
linux at arm.linux.org.uk
Fri May 21 17:35:29 EDT 2010
On Fri, May 21, 2010 at 04:17:28PM -0500, H Hartley Sweeten wrote:
> Hard to tell, but it is very noticeable just doing something like 'dmesg'
> on the console.
> I first get some random number of characters, anywhere from none to something
> less than a full line (80 characters). Then there is a noticeable pause. This
> is followed by some random length block of data, another pause, then the
> remainder of the output.
> I'm not sure if it will help but the 'dmesg' output on my system is 7,472
> It might have nothing to do with the pl010 driver itself. I just don't know
> where to start digging in the driver stack to figure out where the pauses are
> coming from.
I assume that this isn't a new problem, but something that's always
The function in pl010 which loads the characters into the TX fifo is
pl010_tx_chars(). Note that without XON/XOFF flow control,
uap->port.x_char will always be zero.
The loop which loads the FIFO is:
count = uap->port.fifosize >> 1;
writel(xmit->buf[xmit->tail], uap->port.membase + UART01x_DR);
xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
} while (--count > 0);
and the thinking here is that the hardware gives us an interrupt when the
FIFO drops below half-full, so we load the FIFO up with up to half the
maximum number of characters at each interrupt.
In the case of ARM's PL010, that's a 16 byte FIFO - but yours may be
a different depth. If it's smaller, the driver needs to know that so
it doesn't overrun the FIFO. I'd suggest ensuring that 'fifosize' is
correct for your implementation.
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