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Date:   Thu, 21 Jan 2021 11:23:56 +0100
From:   Johan Hovold <johan@kernel.org>
To:     Manivannan Sadhasivam <mani@kernel.org>
Cc:     johan@kernel.org, gregkh@linuxfoundation.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        patong.mxl@gmail.com, linus.walleij@linaro.org,
        mchehab+huawei@kernel.org, angelo.dureghello@timesys.com
Subject: Re: [PATCH v5 3/3] usb: cdc-acm: Ignore Exar XR21V141X when serial
 driver is built
Message-ID: <YAlWPF0vn/zFRsQu@hovoldconsulting.com>
References: <20201122170822.21715-1-mani@kernel.org>
 <20201122170822.21715-4-mani@kernel.org>
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On Sun, Nov 22, 2020 at 10:38:22PM +0530, Manivannan Sadhasivam wrote:
> From: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> 
> The RTS/CTS line discipline for this device doesn't follow
> the standard. So, in order to properly support TX, a separate
> driver is needed.

This took a bit of effort to understand, but I think I know what you're
referring to now. In ACM mode the device has RTS/CTS flow control
enabled, which means TX will be disabled in case CTS isn't wired up
correctly.

It as nothing to do with line disciplines though.

> Ensure that cdc_acm will ignore it during probe time, if the
> Kernel is built with support for it.
>
> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>

> ---
>  drivers/usb/class/cdc-acm.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> index 30ef946a8e1a..719829e6b6db 100644
> --- a/drivers/usb/class/cdc-acm.c
> +++ b/drivers/usb/class/cdc-acm.c
> @@ -1890,6 +1890,12 @@ static const struct usb_device_id acm_ids[] = {
>  	},
>  #endif
>  
> +#if IS_ENABLED(CONFIG_USB_SERIAL_XR)
> +	{ USB_DEVICE(0x04e2, 0x1410),   /* Ignore XR21V141X USB to Serial converter */
> +	.driver_info = IGNORE_DEVICE,
> +	},
> +#endif
> +
>  	/*Samsung phone in firmware update mode */
>  	{ USB_DEVICE(0x04e8, 0x685d),
>  	.driver_info = IGNORE_DEVICE,

I've rewritten the commit message and applied this one now.

Johan