[PATCH] serial: 8250: Fix THRE flag usage for CAP_MINI

[Andy Shevchenko]

On Mon, 2017-06-26 at 14:59 +0100, Phil Elwell wrote:
> The BCM2835 MINI UART has non-standard THRE semantics. Conventionally
> the bit means that the FIFO is empty (although there may still be a
> byte in the transmit register), but on 2835 it indicates that the FIFO
> is not full. This causes interrupts after every byte is transmitted,
> with the FIFO providing some interrupt latency tolerance.
> 
> A consequence of this difference is that the usual strategy of writing
> multiple bytes into the TX FIFO after checking THRE once is unsafe.
> In the worst case of 7 bytes in the FIFO, writing 8 bytes loses all
> but the first since by then the FIFO is full.
> 
> There is an HFIFO ("Hidden FIFO") capability that causes the transmit
> loop to terminate when both THRE and TEMT are set, i.e. when the TX
> block is completely idle. This is unnecessarily cautious, potentially
> causing gaps in transmission.
> 
> Add a new conditional to the transmit loop, predicated on CAP_MINI,
> that exits when THRE is no longer set (the FIFO is full). This allows
> the FIFO to fill quickly but subsequent writes are paced by the
> transmission rate.
> 

>  		if ((up->capabilities & UART_CAP_HFIFO) &&
>  		    (serial_in(up, UART_LSR) & BOTH_EMPTY) !=
> BOTH_EMPTY)
>  			break;
> +		/* The BCM2835 MINI UART THRE bit is really a not-
> full bit. */
> +		if ((up->capabilities & UART_CAP_MINI) &&
> +		    !(serial_in(up, UART_LSR) & UART_LSR_THRE))
> +			break;

Might this end up with two sequential read of LSR?

-- 
Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Intel Finland Oy