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Date:   Tue, 30 Jun 2020 18:55:12 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Daniel Gutson <daniel@eclypsium.com>
Cc:     Derek Kiernan <derek.kiernan@xilinx.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        linux-kernel@vger.kernel.org, Richard Hughes <hughsient@gmail.com>,
        Alex Bazhaniuk <alex@eclypsium.com>
Subject: Re: [PATCH] SPI LPC information kernel module
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On Tue, Jun 30, 2020 at 01:18:28PM -0300, Daniel Gutson wrote:
> On Tue, Jun 30, 2020 at 12:28 PM Greg Kroah-Hartman <
> gregkh@linuxfoundation.org> wrote:
> 
> > On Tue, Jun 30, 2020 at 11:42:58AM -0300, Daniel Gutson wrote:
> > > On Tue, Jun 30, 2020 at 5:56 AM Greg Kroah-Hartman <
> > > gregkh@linuxfoundation.org> wrote:
> > >
> > > > On Mon, Jun 29, 2020 at 07:59:32PM -0300, Daniel Gutson wrote:
> > > > > This kernel module exports configuration attributes for the
> > > > > system SPI chip.
> > > > > This initial version exports the BIOS Write Enable (bioswe),
> > > > > BIOS Lock Enable (ble), and the SMM Bios Write Protect (SMM_BWP)
> > > > > fields of the Bios Control register. The idea is to keep adding more
> > > > > flags, not only from the BC but also from other registers in
> > following
> > > > > versions.
> > > > >
> > > > > The goal is that the attributes are avilable to fwupd when SecureBoot
> > > > > is turned on.
> > > > >
> > > > > A technical note: I check if *ppos == BUFFER_SIZE in the reading
> > function
> > > > > to exit early and avoid an extra access to the HW, for example when
> > using
> > > > > the 'cat' command, which causes two read operations.
> > > >
> > > > Why not use the simple_* functions which should prevent that type of
> > > > thing?
> > > >
> > >
> > > a hint please? I don't see how to do it with simple_read_from_buffer, I
> > > need to return in the read fop the amount of read bytes, but don't know
> > > how to mark EOF. Because of that, 'cat' reads again just for me to tell
> > it
> > > there's nothing else to read.
> >
> > That's fine, the kernel does not tell userspace "EOF", that is up to the
> > libc to determine.  If you read the data from the hardware once, and
> > keep it in your buffer, simple_read_from_buffer() will handle all of
> > that logic for you, please let it do that.
> >
> 
> The only way I see to do this is to dynamically allocate the buffer in the
> open fop, in order to avoid concurrency issues.
> Is this correct?

Or use a lock, depends on what you want to do here.

But sysfs should handle all of this for you, when you switch to it.

> > > We debated this but didn't find a better match, since cpu/arch-specific
> > > seemed too core to put informational drivers.
> > > Do you have a suggestion?
> >
> > Make it explicitly hardware specific in your userspace location.
> >
> 
> What do you mean by "your userspace location"?

Where your files show up to userspace.  sysfs is a hierarchy, don't put
hardware-specific stuff at the "root" of it, otherwise that doesn't make
any sense.  Look at what is there today for examples of what to do.


good luck!

greg k-h