linux-kernel - Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

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Date:	Wed, 27 Aug 2014 21:54:33 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Tony Lindgren <tony@...mide.com>
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	balbi@...com, Vinod Koul <vinod.koul@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

On 08/21/2014 08:44 PM, Tony Lindgren wrote:
>>> Also, with DMA enabled, looks like omap deeper idle states are
>>> blocked as the DMA stays reserved. After I commented out the
>>> DMA info for my console UART, PM started working.
>>
>> Hmm. This would explain something. This would mean that I should cancel
>> the RX DMA transfer in the PM-suspend routine. Let me see how that
>> works.
> 
> OK and if the DMA works with PM, then I don't see why we would not
> want to have it automatically enabled.

I re-did that part where the registers are restored. Mostly for that
reason to use function in runtime_resume() as in set_termios(). I think
that is cute :) _And_ if somebody changes here something and breaks it
then it doesn't work with and without runtime-pm. It looks like the
omap-serial doesn't restore the XON1 & XOFF1 registers.

While at it I made sure that it works as good as I could and that means:

core_pwrdm
(ON),OFF:182,RET:21,INA:131,ON:335,RET-LOGIC-OFF:0,RET-MEMBANK1-OFF:0,RET-MEMBANK2-OFF:0

The core off part with DMA looks like a no no:
I #if 0 the block in where it assigned up.dma. With this I hit
core-off. Step two was

|static void omap8250_update_scr(struct uart_8250_port *up,
|                 struct omap8250_priv *priv)
|{
|serial_out(up, UART_OMAP_SCR, priv->scr | OMAP_UART_SCR_DMAMODE_CTL);
|serial_out(up, UART_OMAP_SCR, priv->scr | OMAP_UART_SCR_DMAMODE_CTL |
OMAP_UART_SCR_DMAMODE_1);
|serial_out(up, UART_OMAP_SCR, priv->scr | |OMAP_UART_SCR_DMAMODE_CTL);
|serial_out(up, UART_OMAP_SCR, priv->scr);
|}

which means I just enable DMA mode in UART and disable it. No DMA
operations were performed.
With this change I see a lost character now and then which means the
UART-IP goes into off and loses its context. Good. However I don't see
core off anymore. This looks like a bug beyond my responsibilities :)

I added code to cancel & and start DMA transfers in runtime suspend
callbacks.
However core-off with DMA won't work. I think we could document this in
the binding document. What do you think?

> BTW, looks like the ports move around now though. If set a port
> to disabled with status = "disabled"; in the .dts file, you'll get
> a different console which does not happen with omap-serial I believe.

You a right. I fixed it in the 8250-core code.

> Regards,
> 
> Tony

Sebastian
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