linux-kernel - Re: [PATCH v13 0/4] userspace MHI client interface driver

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Message-ID: <20201202043839.GL807@yoga>
Date:   Tue, 1 Dec 2020 22:38:39 -0600
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Hemant Kumar <hemantk@...eaurora.org>,
        manivannan.sadhasivam@...aro.org, gregkh@...uxfoundation.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        jhugo@...eaurora.org, bbhatt@...eaurora.org,
        loic.poulain@...aro.org, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, Kalle Valo <kvalo@...eaurora.org>
Subject: Re: [PATCH v13 0/4] userspace MHI client interface driver

On Tue 01 Dec 13:29 CST 2020, Jakub Kicinski wrote:

> On Fri, 27 Nov 2020 19:26:02 -0800 Hemant Kumar wrote:
> > This patch series adds support for UCI driver. UCI driver enables userspace
> > clients to communicate to external MHI devices like modem and WLAN. UCI driver
> > probe creates standard character device file nodes for userspace clients to
> > perform open, read, write, poll and release file operations. These file
> > operations call MHI core layer APIs to perform data transfer using MHI bus
> > to communicate with MHI device. Patch is tested using arm64 based platform.
> 
> Wait, I thought this was for modems.
> 

No, this allows exposing particular channels from any type of MHI
devices.

For modems there is a legacy control path that uses UCI. But data
traffic, (non-legacy) modem control signals and e.g the bearer of GPS
data uses in-kernel drivers that are already in place.

> Why do WLAN devices need to communicate with user space?

They normally don't, all WLAN operations are dealt with within the
kernel. The use case that comes to mind for UCI when it comes to WiFi
products is to avoid implementing the Qualcomm debug (diag) protocol in
the kernel.

As such I think saying that it can be used to communicate with modem or
WLAN devices is misleading. Because while it could be done, it is only
used for dealing with optional side-band services on such products - not
the actual WiFi and modem functionality.

Regards,
Bjorn