linux-kernel - Re: [PATCH v4 5/9] spi: spi-mem: Mark dummy transfers by setting dummy

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Message-ID: <20201218204414.GG5333@sirena.org.uk>
Date:   Fri, 18 Dec 2020 20:44:14 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Pratyush Yadav <p.yadav@...com>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        thierry.reding@...il.com, jonathanh@...dia.com, robh+dt@...nel.org,
        lukas@...ner.de, bbrezillon@...nel.org,
        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 v4 5/9] spi: spi-mem: Mark dummy transfers by setting
 dummy_data bit

On Fri, Dec 18, 2020 at 08:41:02PM +0000, Mark Brown wrote:
> On Sat, Dec 19, 2020 at 12:49:38AM +0530, Pratyush Yadav wrote:

> > Anyway, if the SPI maintainers think this is worth it, I won't object.

> This gets kind of circular, for me it's a question of if there's some
> meaningful benefit from using the feature vs the cost to support it and
> from the sounds of it we don't have numbers on the benefits from using
> it at present.

...although I do have to say looking at the implementation that the cost
seems low, it's just a flag set on an existing transfer.  The only issue
is if we'd get more win from coalesing the entire transaction (or entire
transmit) into a single transfer that could be DMAed and/or requires
fewer trips through the stack which does make it seem like an unclear
tradeoff from the point of view of client drivers

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