Re: [RFC PATCH 8/9] surface_aggregator: Add DebugFS interface

[Maximilian Luz]

On 9/23/20 6:14 PM, Greg Kroah-Hartman wrote:
> On Wed, Sep 23, 2020 at 05:15:10PM +0200, Maximilian Luz wrote:
[...]

> > +// SPDX-License-Identifier: GPL-2.0-or-later
>
> Are you sure about -or-later?  I have to ask.

Fairly, unless there are any complications with integration of this code
that I'm not aware of.

> And no copyright line?

Forgot to add that, sorry. Will add it for the next version. That's also
the case for all other files.

[...]

> > +
> > +out:
> > +	// always try to set response-length and status
> > +	tmp = put_user(rsp.length, &r->response.length);
> > +	if (!ret)
> > +		ret = tmp;
>
> Is that the correct error to return if put_user() fails?  Hint, I don't
> think so...

So the -EFAULT returned by put_user should have precedence? I was aiming
for "in case it fails, return with the first error".

[...]

> > +static long ssam_dbg_device_ioctl(struct file *file, unsigned int cmd,
> > +				    unsigned long arg)
> > +{
> > +	switch (cmd) {
> > +	case SSAM_DBG_IOCTL_GETVERSION:
> > +		return ssam_dbg_if_getversion(file, arg);
>
> Not needed, please drop.
>
> > +
> > +	case SSAM_DBG_IOCTL_REQUEST:
> > +		return ssam_dbg_if_request(file, arg);
> > +
> > +	default:
> > +		return -ENOIOCTLCMD;
>
> Wrong error value.

I assume -ENOTTY would be correct/preferred then? Kernel doc suggests
that either one of the two would be correct and essentially result in
the same behavior.

[...]

> Listen, I'm all for doing whatever you want in debugfs, but why are you
> doing random ioctls here?  Why not just read/write a file to do what you
> need/want to do here instead?

Two reasons, mostly: First, the IOCTL allows me to execute requests in
parallel with just one open file descriptor and not having to maintain
some sort of back-buffer to wait around until the reader gets to reading
the thing. I've used that for stress-testing the EC communication in the
past, which had some issues (dropping bytes, invalid CRCs, ...) under
heavy(-ish) load. Second, I'm considering adding support for events to
this device in the future by having user-space receive events by reading
from the device. Events would also be enabled or disabled via an IOCTL.
That could be implemented in a second device though. Events were also my
main reason for adding a version to this interface: Discerning between
one that has event support and one that has not.

> And again, no versioning, that is never needed.

Got it, will drop that.

[...]

> > +static void ssam_dbg_device_release(struct device *dev)
> > +{
> > +	// nothing to do
>
> That's a lie, and the old documentation would allow me to make fun of
> you for trying to work around the kernel's error messages here.
>
> But I'll be nice and just ask, why do you think it is ok to work around
> a message that someone has spent a lot of time and energy to provide to
> you, saying that you are doing something wrong, by ignoring that and
> providing an empty function?  Not kind...

Sorry about that, but may get a pointer to that particular message? This
setup has been pretty much copied from existing kernel drivers (see
/drivers/platform/x86/intel_pmc_core_pltdrv.c for one) and I thought
that I can get around having to dynamically allocate a platform device
since it's guaranteed to be only there once.

There was no workaround or unkindness of any sorts intended.

> > +}
> > +
> > +static struct platform_device ssam_dbg_device = {
> > +	.name = SSAM_DBG_DEVICE_NAME,
> > +	.id = PLATFORM_DEVID_NONE,
> > +	.dev.release = ssam_dbg_device_release,
> > +};
>
> Dynamic structures that are static are, well, wrong :)

I assume the correct way would be to allocate the device dynamically and
this holds for all devices?

Sorry if I'm asking such basic questions, but I have not found anything
regarding this in the documentation, although I have to confess that I
only skimmed over a larger part, so that's very likely my fault.

> I appreciate the initiative by creating a fake platform device and
> driver to bind to that device.  But I don't think any of it is needed at
> all, you have made your work a lot harder than you needed to here.  This
> whole file can be _much_ smaller and simpler and not abuse the kernel
> apis so badly :)

So just tack it onto the core driver? My intention was to keep it a bit
more separate from the core, but adding it directly would indeed reduce
the amount of code.

Thanks,
Max