linux-kernel - Re: [PATCH] serial: imx: disable DMA for RS-485 on i.MX6 SMP

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Date:   Wed, 21 Jun 2017 16:05:56 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Fabio Estevam <festevam@...il.com>
Cc:     linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Fabio Estevam <fabio.estevam@....com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Fugang Duan <fugang.duan@....com>
Subject: Re: [PATCH] serial: imx: disable DMA for RS-485 on i.MX6 SMP

Hi Fabio,

On Tue, Jun 20, 2017 at 01:13:18PM -0300, Fabio Estevam wrote:
> The subject gives the impression that the DMA will only be disabled
> for RS485, but the impact of this change is wider.
> 
> For example: if I have a mx6q system with a Bluetooth serial
> connection I can no longer use DMA with your change applied.
> 
> Ideally we should fix the RS485 DMA bug. If that is not possible, then
> at least we need to restrict this change to the RS485 case.
> 
> Maybe we need to pass "linux,rs485-enabled-at-boot-time" in device
> tree and then use this property to deceide if DMA will be enabled or
> not:
> 
> if (!uart_console(port) && !sport->dma_is_inited && !sport->rs485_enabled)

I'd also prefer fixing the underlying problem.
If you take a look at the DMA parts in drivers/tty/serial/imx.c, can you
spot anything SMP-unsafe?
By the way, can you get your hands on an i.MX6Q board with RS-485 to
reproduce it?

We could try for a few more weeks to find the bug and if we don't find
it, adding this rs485-enabled-at-boot-time property sounds good!

Thanks,
Clemens