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Date:   Tue, 4 Apr 2023 12:41:19 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Jaewon Kim <jaewon02.kim@samsung.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        Andi Shyti <andi@etezian.org>,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Rob Herring <robh+dt@kernel.org>, linux-spi@vger.kernel.org,
        linux-samsung-soc@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Chanho Park <chanho61.park@samsung.com>
Subject: Re: [PATCH 1/3] spi: s3c64xx: support spi polling mode using
 devicetree
Message-ID: <aca77fe7-5fed-4ba1-ab28-8b66281224d2@sirena.org.uk>
References: <20230404060011.108561-1-jaewon02.kim@samsung.com>
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On Tue, Apr 04, 2023 at 08:17:13PM +0900, Jaewon Kim wrote:
> On 23. 4. 4. 19:54, Mark Brown wrote:
> > On Tue, Apr 04, 2023 at 03:00:09PM +0900, Jaewon Kim wrote:

> >> This patch adds new 'samsung,spi-polling' property to support polling mode.
> >> In some environments, polling mode is required even if DMA is supported.
> >> Changed it to support not only with quick but also optinally with
> >> devicetree.

> > Why would this be required if we can use DMA?  If this is a performance
> > optimisation for small messages the driver should just work out when to
> > choose PIO over DMA like other drivers do.  It is hard to see this as a
> > hardware property which should be configured via DT.

> We are providing a VM environment in which several Guest OSs are running.
> If Host OS has DMA, GuestOS should use SPI as polling mode.

This sounds like some sort of virtualised environment with passthrough?
If that's the case then the host OS will be in control of the device
tree provided to the guest so it simply shouldn't be describing the DMA
configuration if it doesn't want the guest to use DMA for some reason.
There's no value in describing the DMA the guest shouldn't use then
providing an additional property telling the guest not to pay attention
to the DMA when we could simply not do the first step.

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