linux-kernel - Re: [PATCH 2/2] spi: spi-qcom-qspi: Add mem_ops to avoid PIO for badly sized reads

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Date:   Tue, 25 Jul 2023 11:30:30 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH 2/2] spi: spi-qcom-qspi: Add mem_ops to avoid PIO for
 badly sized reads

Hi,

On Tue, Jul 25, 2023 at 11:24 AM Mark Brown <broonie@...nel.org> wrote:
>
> On Tue, Jul 25, 2023 at 11:02:27AM -0700, Douglas Anderson wrote:
> > In the patch ("spi: spi-qcom-qspi: Fallback to PIO for xfers that
> > aren't multiples of 4 bytes") we detect reads that we can't handle
> > properly and fallback to PIO mode. While that's correct behavior, we
> > can do better by adding "spi_controller_mem_ops" for our
> > controller. Once we do this then the caller will give us a transfer
> > that's a multiple of 4-bytes so we can DMA.
>
> This is more of an optimisation for the case where we're using flash -
> if someone has hung some other hardware off the controller (which seems
> reasonable enough if they don't need it for flash) then we'll not use
> the mem_ops.

Right. That's why I also have the first patch in the series too. The
first patch ensures correctness and the second patch makes things more
optimal for when we're using flash. Do you want me to re-submit the
patch with wording that makes this more obvious?

Note that it's pretty likely someone wouldn't use this SPI controller
for anything other than SPI flash. While it's theoretically possible,
the controller is intended for SPI flash (supports dual and quad SPI
modes) and is only half duplex.

-Doug