[PATCH 05/15] tty: serial: Add 8250-core based omap driver

[Lennart Sorensen]

On Fri, Aug 15, 2014 at 09:27:59PM +0200, Sebastian Andrzej Siewior wrote:
> If you want to change this to reduce the gap, then you have first
> change 8250 core code. Currently it waits until the shift register is
> empty.

Oh the 8250 normally works this way?  I didn't know that.

> On the other hand if you use DMA then it can handle transfers > 64bytes
> in one go and you can start transfers while the FIFO is not completely
> empty.

You can dma more than the fifo size?

> If you use DMA. You program one transfer says 100 bytes. You get an
> dma-transfer complete once the 100 bytes are transfered which means the
> FIFO has 63 bytes. From this point on you could enqueue the next
> transfer with say another 100 bytes. In that scenario you don't see the
> gap.
> 
> You get only to the gap if you use the non-DMA mode (and not UARTs
> support DMA). In that case, yes waiting till there only 16 bytes before
> starting the refill would make sense if you want to utilize the port by
> 100%. But as I said in 0/15, you need to teach the core this first.
> Otherwise it will return doing nothing until the shift register is
> empty (i.e. until the FIFO is completely empty).

Well if DMA takes care of it, and the normal 8250 is already like this,
then I suppose it is already better than the typical case.

> There is patch in Greg's tty tree already where you are able to
> configure the RX trigger level. We could wire this up once we agree
> which levels we want support. The OMAP supports all levels from 1…63.

All? or just every 4 (that's what I just read in the DRA7xx docs).

> Yes, true. However this is only an issue without HW control. With DMA
> the buffer is slightly larger. The DMA engine starts the transfer on
> its own once there 48 bytes in the FIFO (except in the few cases where
> it does not).

That's nice of it.  I will have to give this a try.

-- 
Len Sorensen