linux-kernel - Re: [PATCH v2 2/4] soc: qcom: aoss: Add debugfs interface for sending messages

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Message-ID: <20230811233228.GT1428172@hu-bjorande-lv.qualcomm.com>
Date:   Fri, 11 Aug 2023 16:32:28 -0700
From:   Bjorn Andersson <quic_bjorande@...cinc.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Chris Lew <quic_clew@...cinc.com>,
        Alex Elder <elder@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-remoteproc@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] soc: qcom: aoss: Add debugfs interface for
 sending messages

On Fri, Aug 11, 2023 at 11:01:50PM +0200, Andrew Lunn wrote:
> > +static ssize_t qmp_debugfs_write(struct file *file, const char __user *userstr,
> > +				 size_t len, loff_t *pos)
> > +{
> > +	struct qmp *qmp = file->private_data;
> > +	char buf[QMP_MSG_LEN];
> > +	int ret;
> > +
> > +	if (!len || len >= QMP_MSG_LEN)
> > +		return -EINVAL;
> > +
> > +	if (copy_from_user(buf, userstr, len))
> > +		return -EFAULT;
> > +	buf[len] = '\0';
> > +
> > +	ret = qmp_send(qmp, buf);
> > +	if (ret < 0)
> > +		return ret;
> 
> Sorry, but you still appear to be sending binary blobs from userspace
> to the firmware. This is not liked.
> 

As mentioned in the cover letter, I do recognize your concern here. I
don't see it as a realistic way to work around the kernel for reasons of
being proprietary - given that we don't have debugfs mounted in the vast
majority of product.

I do however recognize the benefit of this interface for fellow upstream
engineers.

> The documentation you pointed to has three commands. Please implement
> three debugfs files, one per command.
> 

The documentation pointed to has 4 classes ("class"), but this is not
the full set, each class has N resources ("res") and each resource has a
varying value space - "off", "mol", "enabled", "disabled", "max" to take
the examples from the documentation, other classes takes integers as
argument. Some classes has a fourth key...

Further more, the list of classes, resources and values varies from
target to target.

We're composing the lists of commands, but I'm not sure that it will be
feasible to spell out all the valid commands, on a per-target basis.
It is just a debug feature, I don't want it to take up a significant
portion of the driver.

The alternative is to continue carrying this as an out-of-tree patch,
the only people suffering from that are the ones working exclusively
in on the upstream kernel.

Regards,
Bjorn