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

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Message-ID: <20201201120302.474d4c9b@kicinski-fedora-pc1c0hjn.DHCP.thefacebook.com>
Date:   Tue, 1 Dec 2020 12:03:02 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jeffrey Hugo <jhugo@...eaurora.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,
        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, 1 Dec 2020 12:40:50 -0700 Jeffrey Hugo wrote:
> On 12/1/2020 12:29 PM, 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.
> > 
> > Why do WLAN devices need to communicate with user space?
> >   
> 
> Why does it matter what type of device it is?  Are modems somehow unique 
> in that they are the only type of device that userspace is allowed to 
> interact with?

Yes modems are traditionally highly weird and require some serial
device dance I don't even know about.

We have proper interfaces in Linux for configuring WiFi which work
across vendors. Having char device access to WiFi would be a step 
back.

> However, I'll bite.  Once such usecase would be QMI.  QMI is a generic 
> messaging protocol, and is not strictly limited to the unique operations 
> of a modem.
> 
> Another usecase would be Sahara - a custom file transfer protocol used 
> for uploading firmware images, and downloading crashdumps.

Thanks, I was asking for use cases, not which proprietary vendor
protocol you can implement over it.

None of the use cases you mention here should require a direct FW -
user space backdoor for WLAN.

> Off the top of my head, this driver is useful for modems, wlan, and AI 
> accelerators.

And other Qualcomm products are available as well :/

Kernel is supposed to create abstract interfaces for user space to
utilize. I will never understand why kernel is expected to be in
business of shipping this sort of vendor backdoors :/