linux-kernel - Re: [PATCH 1/3] spi: s3c64xx: support spi polling mode using devicetree

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Date:   Tue, 4 Apr 2023 20:17:13 +0900
From:   Jaewon Kim <jaewon02.kim@...sung.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Andi Shyti <andi@...zian.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Rob Herring <robh+dt@...nel.org>, linux-spi@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Chanho Park <chanho61.park@...sung.com>
Subject: Re: [PATCH 1/3] spi: s3c64xx: support spi polling mode using
 devicetree

Hello Mark,

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.

In order to support s3c64xx in a DMA-less environment, it must be 
separated with a quirk.
However, there is DMA in the Host OS and no DMA in the Guest OS,
it is not correct to separate them with quirk.

I'm considering supporting this systems with DeviceTree rather than qurik.
If 'samsung,spi-polling' looks to be a SW configuration, how about 
'samsung,no-dma'.

This is not to simply change the mode using DeviceTree, but to support 
an environment without DMA.

Thanks

Jaewon Kim