Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation for writel() example

Paul E. McKenney
2022-10-27T18:16:15Z

On Tue, Oct 18, 2022 at 08:33:08PM +0000, Parav Pandit wrote:
> Hi Paul, Will,
>
> > From: Paul E. McKenney <paulmck@kernel.org>
> > Sent: Tuesday, October 18, 2022 1:49 PM
> >
> > On Tue, Oct 18, 2022 at 11:05:55AM +0100, Will Deacon wrote:
> > > On Mon, Oct 17, 2022 at 10:55:00PM +0200, Arnd Bergmann wrote:
> > > > On Mon, Oct 10, 2022, at 12:13 PM, Parav Pandit wrote:
> > > > > The cited commit describes that when using writel(), explcit wmb()
> > > > > is not needed. wmb() is an expensive barrier. writel() uses the
> > > > > needed platform specific barrier instead of expensive wmb().
> > > > >
> > > > > Hence update the example to be more accurate that matches the
> > > > > current implementation.
> > > > >
> > > > > commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA
> > vs.
> > > > > MMIO ordering example")
> > > > >
> > > > > Signed-off-by: Parav Pandit <parav@nvidia.com>
> > > >
> > > > I have no objections, though I still don't see a real need to change
> > > > the wording here.
> > >
> > > FWIW, I also don't think this change is necessary. If anything, I'd
> > > say we'd be better off _removing_ the text about writel from this
> > > section and extending the reference to the "KERNEL I/O BARRIER
> > > EFFECTS" section, as you could make similar comments about e.g.
> > > readb() and subsequent barriers.
> > >
> > > For example, something like the diff below.
> >
> > I do like this change, but we might be dealing with two different groups of
> > readers. Will and Arnd implemented significant parts of the current
> > MMIO/DMA ordering infrastructure. It is thus quite possible that wording
> > which suffices to remind them of how things work might or might not help
> > someone new to Linux who is trying to figure out what is required to make
> > their driver work.
> >
> > The traditional resolution of this sort of thing is to provide the
> > documentation to a newbie and take any resulting confusion seriously.
> >
> > Parav, thoughts?
>
> I am ok with the change from Will that removes the writel() description.
> However, it removes useful short description from the example of "why" writel() is used.
> This is useful for newbie and experienced developers both.
>
> So how about below additional change on top of Will's change?
> This also aligns to rest of the short C comments in this example pseudo code.
>
> If ok, I will take Will's and mine below change to v5.
>
> index 4d24d39f5e42..5939c5e09570 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1919,7 +1919,9 @@ There are some more advanced barrier functions:
> /* assign ownership */
> desc->status = DEVICE_OWN;
>
> - /* notify device of new descriptors */
> + /* Make descriptor status visible to the device followed by
> + * notify device of new descriptors
> + */
> writel(DESC_NOTIFY, doorbell);

Hearing no objections, please proceed.

Thanx, Paul