linux-kernel - Re: [PATCH] spi: Skip zero-length transfers in spi_transfer_one

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Date:   Fri, 12 Feb 2021 13:57:24 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Robin Murphy <robin.murphy@....com>,
        Phil Elwell <phil@...pberrypi.com>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] spi: Skip zero-length transfers in spi_transfer_one_message()

Hi Mark,

On Fri, Feb 12, 2021 at 1:55 PM Mark Brown <broonie@...nel.org> wrote:
> On Fri, Feb 12, 2021 at 01:48:21PM +0100, Nicolas Saenz Julienne wrote:
> > On Fri, 2021-02-12 at 12:31 +0000, Mark Brown wrote:
> > > On Thu, Feb 11, 2021 at 07:08:20PM +0100, Nicolas Saenz Julienne wrote:
>
> > > > -         if (xfer->tx_buf || xfer->rx_buf) {
> > > > +         if ((xfer->tx_buf || xfer->rx_buf) && xfer->len) {
>
> > > I think the issue here is more that some users were passing in buffers
> > > with zero length transfers, the above check was already intended to
> > > catch this case but was working on the assumption that if there was
> > > nothing to transfer then no buffer would be provided.
>
> > Fair enough, maybe it makes sense to move the check into __spi_validate() and
> > propagate an error upwards?
>
> No, I think it's fine - there's probably some sensible use case with
> drivers reusing a statically allocated transfer/buffer set for multiple
> operations and just tweaking the length as needed which seems a bit
> weird but I can't think of a reason not to allow it.  Your patch is
> currently queued, all being well it'll get tested & pushed out later
> today.

Aren't the zero-length transfers also used to do tricks with the CS signal,
e.g. combined with cs_change?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds