linux-kernel - Re: [PATCH v3 2/2] drivers/serial: Add driver for Aspeed virtual UART

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Date:   Tue, 2 May 2017 17:15:36 +0930
From:   Joel Stanley <joel@....id.au>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>, Jeremy Kerr <jk@...abs.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>
Subject: Re: [PATCH v3 2/2] drivers/serial: Add driver for Aspeed virtual UART

On Tue, Apr 11, 2017 at 9:45 PM, Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Mon, Apr 10, 2017 at 7:04 AM, Joel Stanley <joel@....id.au> wrote:
>> From: Jeremy Kerr <jk@...abs.org>
>>
>> This change adds a driver for the 16550-based Aspeed virtual UART
>> device. We use a similar process to the of_serial driver for device
>> probe, but expose some VUART-specific functions through sysfs too.
>>
>> The VUART is two UART 'front ends' connected by their FIFO (no actual
>> serial line in between). One is on the BMC side (management controller)
>> and one is on the host CPU side.
>>
>> This driver is for the BMC side. The sysfs files allow the BMC
>> userspace, which owns the system configuration policy, to specify at
>> what IO port and interrupt number the host side will appear to the host
>> on the Host <-> BMC LPC bus. It could be different on a different system
>> (though most of them use 3f8/4).
>>
>> OpenPOWER host firmware doesn't like it when the host-side of the
>> VUART's FIFO is not drained. This driver only disables host TX discard
>> mode when the port is in use. We set the VUART enabled bit when we bind
>> to the device, and clear it on unbind.
>>
>> We don't want to do this on open/release, as the host may be using this
>> bit to configure serial output modes, which is independent of whether
>> the devices has been opened by BMC userspace.
>
>> +static void aspeed_vuart_set_enabled(struct aspeed_vuart *vuart, bool enabled)
>> +{
>> +       u8 reg;
>> +
>> +       reg = readb(vuart->regs + ASPEED_VUART_GCRA);
>
>> +       reg &= ~ASPEED_VUART_GCRA_VUART_EN;
>> +       if (enabled)
>> +               reg |= ASPEED_VUART_GCRA_VUART_EN;
>
> Usually the pattern is
> if (something)
>  set x bit;
> else
>  clear x bit;
>
> It would make it one operation in any case and a bit more understandable.

I have made these ordering changes you requested.

>
>> +       port.port.irq = irq_of_parse_and_map(np, 0);
>
> The benefit of use platform_get_irq() is to get rid of some OF specific headers.

It's an of driver, and this function does exactly what we require. I
have left it in for v4.

Cheers,

Joel