linux-kernel - Re: [PATCH 3/3] tty/serial: at91: fix hardware handshake when DMA is not used

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Message-ID: <20160907165918.xog3poqstw434r2j@piout.net>
Date:   Wed, 7 Sep 2016 18:59:18 +0200
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Richard Genoud <richard.genoud@...il.com>
Cc:     Cyrille Pitchen <cyrille.pitchen@...el.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 3/3] tty/serial: at91: fix hardware handshake when DMA is
 not used

Hi,

On 07/09/2016 at 18:13:24 +0200, Richard Genoud wrote :
> Commit 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when
> hardware handshake is enabled") broke the hardware handshake when
> DMA is not used.
> 
> So, here's a summary:
> If DMA is NOT USED the mode should be ATMEL_US_USMODE_NORMAL because the
> controller can't drive the RTS pin (there's no way for it to know when
> we can't receive data anymore).
> 

You forgot that the PDC can properly drive the RTS pin so all the
platforms that are using the PDC will break after your patch.

Also, I believe the controller is able to drive the RTS and CTS pins
when simply using PIOs (it knows when one complete character has been
received and US_RHR has not yet been read). However, I didn't test.
Maybe someone at Atmel can confirm.

I think that 5be605ac9af9 is doing the right thing but I still can be
convince otherwise ;).

-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com