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

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Date:   Fri, 23 Jun 2017 09:26:14 +0000
From:   "Schenk, Gavin" <G.Schenk@...elmann.de>
To:     Clemens Gruber <clemens.gruber@...ruber.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>,
        "Fabio Estevam" <fabio.estevam@....com>
Subject: AW: [Customers.Eckelmann] [PATCH] serial: imx: disable DMA for
 RS-485 on i.MX6 SMP

Hi,

> 
> DMA support for half-duplex RS-485 never worked correctly on i.MX6Q/D
> due to an undiscovered SMP-related bug, instead of the real data being
> sent out, the rest of the transmit buffer is sent (xmit->tail jumps over
> xmit->head in imx_transmit_buffer and UART_XMIT_SIZE bytes are sent out)
> More details: https://lkml.org/lkml/2017/1/4/579
> 

We see exactly this issue on our platform, based on i.MX6dl. We use the RS485 mode to implement a Modbus interface. We have another platform based on i.MX25 running the same Linux 4.9.30 code that is not affected!

> Disable it for that configuration until the bug is found and fixed.
> 
> We need to know at probe time if we can enable DMA. (RS-485 could be
> enabled after that). Let's therefore only enable DMA if it is not an
> i.MX6Q/D UART with uart-has-rtscts in the DT and CONFIG_SMP enabled.
> 

Uwe Kleine-König had the idea to disable DMA on the i.MX6dl based platform via dts and this is our current workaround.

Regards
Gavin Schenk

Eckelmann AG
Vorstand: Dipl.-Ing. Peter Frankenbach (Sprecher) Dipl.-Wi.-Ing. Philipp Eckelmann
Dr.-Ing. Marco Münchhof Dr.-Ing. Frank Uhlemann
Vorsitzender des Aufsichtsrats: Hubertus G. Krossa
Stv. Vorsitzender des Aufsichtsrats: Dr.-Ing. Gerd Eckelmann
Sitz der Gesellschaft: Berliner Str. 161, 65205 Wiesbaden, Amtsgericht Wiesbaden HRB 12636
http://www.eckelmann.de