linux-kernel - Re: [PATCH v3] PCI/DOE: Expose the DOE protocols via sysfs

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Date:   Thu, 10 Aug 2023 09:48:50 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Alistair Francis <alistair23@...il.com>, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org, Jonathan.Cameron@...wei.com,
        alex.williamson@...hat.com, christian.koenig@....com,
        kch@...dia.com, logang@...tatee.com, linux-kernel@...r.kernel.org,
        Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH v3] PCI/DOE: Expose the DOE protocols via sysfs

On Thu, Aug 10, 2023 at 09:44:44AM +0200, Lukas Wunner wrote:
> On Thu, Aug 10, 2023 at 07:05:12AM +0200, Greg KH wrote:
> > On Wed, Aug 09, 2023 at 07:28:51PM -0400, Alistair Francis wrote:
> > > v3:
> > >  - Expose each DOE feature as a separate file
> > 
> > But you don't actually have anything in the sysfs files, why not?
> 
> He wants to expose a list of supported protocols.
> 
> He first exposed the list in a single attribute, separated by newlines.
> Which made sense because it allows users to grep for a specific protocol.
> 
> You told him not to expose multiple values in a single attribute.
> So he's exposing the available protocols each in an empty file.
> The file name contains the protocol.
> 
> You got what you asked for. ;)

But that's not what was documented, it should say "empty file",
otherwise this is going to be very odd when people try to read a file
that is marked as readable, but yet returns an error :(

> > > --- a/drivers/pci/doe.c
> > > +++ b/drivers/pci/doe.c
> > > @@ -56,6 +56,10 @@ struct pci_doe_mb {
> > >  	wait_queue_head_t wq;
> > >  	struct workqueue_struct *work_queue;
> > >  	unsigned long flags;
> > > +
> > > +#ifdef CONFIG_SYSFS
> > > +	struct device_attribute *sysfs_attrs;
> > > +#endif
> > 
> > Please don't put #ifdefs in .c files if you can prevent it.  I think
> > this will work just fine if you don't have the #ifdef.  And who would be
> > using pci without sysfs?
> 
> People with space-constrained devices such as routers.

So the extra pointer here is a real problem for them?  And how much
memory are you saving?

> It is perfectly legal to compile a kernel with CONFIG_PCI=y and
> CONFIG_SYSFS=n.

Sure, just not common.

> And it is reasonable not to include code in the kernel which has
> specifically been deselected in the kernel config.

Sure, but not at the expense of a zillion #ifdef lines :)

thanks,

greg k-h