linux-kernel - Re: Serial 8250 DMA Broken on OMAP3630

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Message-ID: <20191009173446.GL5610@atomide.com>
Date:   Wed, 9 Oct 2019 10:34:46 -0700
From:   Tony Lindgren <tony@...mide.com>
To:     Adam Ford <aford173@...il.com>
Cc:     Vignesh Raghavendra <vigneshr@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Douglas Anderson <dianders@...omium.org>,
        Yegor Yefremov <yegorslists@...glemail.com>,
        linux-serial@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Serial 8250 DMA Broken on OMAP3630

* Adam Ford <aford173@...il.com> [191009 14:09]:
> On Wed, Oct 9, 2019 at 8:42 AM Vignesh Raghavendra <vigneshr@...com> wrote:
> >
> > Hi Adam,
> >
> > On 06/10/19 10:34 PM, Adam Ford wrote:
> > > Has anyone else had any issues using the CONFIG_SERIAL_8250_DMA on the OMAP?
> > >
> > > I can use the DMA on the legacy, omap-serial driver, but when I enable
> > > the DMA on the 8250-omap driver, I get missing frames in Bluetooth.
> > >
> > > The older driver seems to have an ISR that seems to address a variety
> > > of items compared to the very tiny ISR for 8250-omap.c.
> > >
> > > I am not exactly sure where to start, but if someone has any
> > > suggestions on how I can troubleshoot, please let me know.  As of now,
> > > I have to disable CONFIG_SERIAL_8250_DMA to get the Bluetooth
> > > connected to UART2 operational on a DM3730 at 3,000,000 baud, but it
> > > appears to work just fine after some patches I just submitted for
> > > handling RTS/CTS.  The legacy omap-serial driver works fine with DMA.
> > >
> >
> > Mainline omap-serial does not support DMA (evident from lack of
> > dmaengine API calls) and therefore is not a regression. So OMAP3 UART
> > DMA was never tested at least with 8250 driver.
> > I am not sure enabling UART DMA on OMAP3 would be a trivial job. We need
> > analyse of whether all erratas workarounds are implemented and see if
> > there any difference wrt DMA integration itself. Do we know if UART DMA
> > ever worked on OMAP3 previously?
> 
> Can we disable CONFIG_SERIAL_8250_DMA from omap2plus_defconfig?   As
> of right now, it's the only way I can use the Bluetooth.  There are
> #ifdef's in the 8250-omap driver, so it looks like some of the hooks
> are there, but if it's incomplete, it seems like we shouldn't enable
> this config option by default yet.

>From what I recall I tested that DMA on omap3 worked fine with runtime
PM for console. Certainly there are issues still remaining though.

If you want to disable dma for a specific port, just delete the
add /delete-property/dma-names for the port in question in your
board specific dts file.

See 8250_omap.c for of_property_count_strings(np, "dma-names").

Regards,

Tony