linux-kernel - Re: [PATCH v3 5/9] spi: spi-mem: Allow masters to transfer dummy cycles directly by hardware

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Message-ID: <20201214162315.GA4880@sirena.org.uk>
Date:   Mon, 14 Dec 2020 16:23:15 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com, robh+dt@...nel.org,
        lukas@...ner.de, bbrezillon@...nel.org, p.yadav@...com,
        tudor.ambarus@...rochip.com, linux-spi@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v3 5/9] spi: spi-mem: Allow masters to transfer dummy
 cycles directly by hardware

On Sat, Dec 12, 2020 at 11:57:15AM +0100, Boris Brezillon wrote:
> Sowjanya Komatineni <skomatineni@...dia.com> wrote:

> > This patch adds a flag SPI_MASTER_USES_HW_DUMMY_CYCLES for the controllers
> > that support transfer of dummy cycles by the hardware directly.

> Hm, not sure this is a good idea. I mean, if we expect regular SPI
> devices to use this feature, then why not, but if it's just for
> spi-mem, I'd recommend implementing a driver-specific exec_op() instead
> of using the default one.

I *have* seen other high speed devices which had padding bits in the
transfer (see regmap's pad_bits feature), I think that corresponds to
flash dummy bits but haven't checked that the hardware support lines up.
I'm not sure it's ever been seen as something that we particularly
needed to speed up with hardware offload though.

> If we go for those core changes, we should at least add a
> ctrl->max_dummy_cycles field so the core can fallback to regular writes
> when the number of dummy cycles in the spi_mem_op exceeds what the
> controller can do.

That seems sensible if there's a risk of controllers being too limited,
which knowing hardware seems likely.

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