linux-kernel - Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs

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Date:   Tue, 21 Dec 2021 08:13:07 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Rafał Miłecki <zajec5@...il.com>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs

On Tue, Dec 21, 2021 at 07:53:32AM +0100, Rafał Miłecki wrote:
> On 21.12.2021 07:45, Greg Kroah-Hartman wrote:
> > On Tue, Dec 21, 2021 at 07:39:24AM +0100, Rafał Miłecki wrote:
> > > On 21.12.2021 07:33, Greg Kroah-Hartman wrote:
> > > > On Mon, Dec 20, 2021 at 09:39:43PM +0100, Rafał Miłecki wrote:
> > > > > Hi Greg,
> > > > > 
> > > > > On 20.12.2021 09:00, Greg Kroah-Hartman wrote:
> > > > > > On Mon, Dec 20, 2021 at 07:47:30AM +0100, Rafał Miłecki wrote:
> > > > > > >     static void nvmem_cell_entry_add(struct nvmem_cell_entry *cell)
> > > > > > >     {
> > > > > > > +	struct device *dev = &cell->nvmem->dev;
> > > > > > > +	int err;
> > > > > > > +
> > > > > > >     	mutex_lock(&nvmem_mutex);
> > > > > > >     	list_add_tail(&cell->node, &cell->nvmem->cells);
> > > > > > >     	mutex_unlock(&nvmem_mutex);
> > > > > > > +
> > > > > > > +	sysfs_attr_init(&cell->battr.attr);
> > > > > > > +	cell->battr.attr.name = cell->name;
> > > > > > > +	cell->battr.attr.mode = 0400;
> > > > > > > +	cell->battr.read = nvmem_cell_attr_read;
> > > > > > > +	err = sysfs_add_bin_file_to_group(&dev->kobj, &cell->battr,
> > > > > > > +					  nvmem_cells_group.name);
> > > > > > 
> > > > > > Why not just use the is_bin_visible attribute instead to determine if
> > > > > > the attribute should be shown or not instead of having to add it
> > > > > > after-the-fact which will race with userspace and loose?
> > > > > 
> > > > > I'm sorry I really don't see how you suggest to get it done.
> > > > > 
> > > > > I can use .is_bin_visible() callback indeed to respect nvmem->root_only.
> > > > 
> > > > Great.
> > > > 
> > > > > I don't understand addig-after-the-fact part. How is .is_bin_visible()
> > > > > related to adding attributes for newly created cells?
> > > > 
> > > > You are adding a sysfs attribute to a device that is already registered
> > > > in the driver core, and so the creation of that attribute is never seen
> > > > by userspace.  The attribute needs to be attached to the device _BEFORE_
> > > > it is registered.
> > > > 
> > > > Also, huge hint, if a driver has to call as sysfs_*() call, something is
> > > > wrong.
> > > > 
> > > > > Do you mean I can
> > > > > avoid calling sysfs_add_bin_file_to_group()?
> > > > 
> > > > Yes.
> > > > 
> > > > > Do you recall any existing example of such solution?
> > > > 
> > > > Loads.
> > > > 
> > > > Just add this attribute group to your driver as a default attribute
> > > > group and the driver core will create it for you if needed.
> > > > 
> > > > Or if you always need it, no need to mess sith is_bin_visible() at all,
> > > > I can't really understand what you are trying to do here at all.
> > > 
> > > Thanks a lot! In nvmem_register() first there is a call to the
> > > device_register() and only later cells get added. I suppose I just have
> > > to rework nvmem_register() order so that:
> > > 1. Cells are collected earlier. For each cell I allocate group attribute
> > 
> > No, add all of the attributes to the device at the beginning before you
> > register it, there's no need to allocate anything.
> 
> If you mean static structures I can't do that, because cells almost
> never are static. They are not known in advance. nvmem allows cells to
> be:
> 1. Specified in OF
> 2. Submitted as list while registering a NVMEM device
> 
> So every cells gets its own structure allocated dynamically. My plan is
> to put bin_attribute in that struct and then create a group collecting
> all those cells.

A device has a driver associated with it, and that driver has default
groups associated with it.  Use that, I am not saying to use static
structures, that is not how the driver model works at all.

thanks,

greg k-h