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References: <20210115211543.33563-1-alcooperx@gmail.com> <20210115211543.33563-3-alcooperx@gmail.com>
 <YAXJRDhS1HXeYaZz@kroah.com>
In-Reply-To: <YAXJRDhS1HXeYaZz@kroah.com>
From:   Al Cooper <al.cooper@broadcom.com>
Date:   Mon, 18 Jan 2021 15:32:57 -0500
Message-ID: <CAGh=XABU=jdLWo8AfSBZPwn+_gDfCNN07cg9a58nxknjjyEfxg@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] serial: 8250: Add new 8250-core based Broadcom STB driver
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Al Cooper <alcooperx@gmail.com>, linux-kernel@vger.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, Jiri Slaby <jirislaby@kernel.org>,
        linux-serial@vger.kernel.org,
        "open list:USB SUBSYSTEM" <linux-usb@vger.kernel.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Rob Herring <robh+dt@kernel.org>
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On Mon, Jan 18, 2021 at 12:45 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Fri, Jan 15, 2021 at 04:15:43PM -0500, Al Cooper wrote:
> > Add a UART driver for the new Broadcom 8250 based STB UART. The new
> > UART is backward compatible with the standard 8250, but has some
> > additional features. The new features include a high accuracy baud
> > rate clock system and DMA support.
> >
> > The driver will use the new optional BAUD MUX clock to select the best
> > one of the four master clocks (81MHz, 108MHz, 64MHz and 48MHz) to feed
> > the baud rate selection logic for any requested baud rate.  This allows
> > for more accurate BAUD rates when high speed baud rates are selected.
> >
> > The driver will use the new UART DMA hardware if the UART DMA registers
> > are specified in Device Tree "reg" property. The DMA functionality can
> > be disabled on kernel boot with the argument:
> > "8250_bcm7271.disable_dma=Y".
>
> Shouldn't that be on a per-device basis, and not a per-driver basis?

There is only one instance of the UART DMA hardware and it gets muxed
to just one of the possible UARTS.

>
> And why would you want to disable this, if you have support for this in
> the DT?  Why not just rely on the DT setting?

The DMA feature is used when the UART is connected to a Bluetooth
controller and the BAUD rate is typically 2-3Mbs. The ability to
easily disable DMA is very useful when debugging BT communication
problems in the field. DT settings could also be used to disable DMA,
but knowing the correct modifications to the "reg" and "reg-names"
properties is a lot more complicated.

Thanks
Al

>
> thanks,
>
> greg k-h

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