linux-kernel - Re: [PATCH] spi: meson-spicc: save pow2 datarate between messages

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Message-ID: <39c2f53b-8f53-ceb1-ae0c-81e5e53d01aa@baylibre.com>
Date:   Wed, 10 Aug 2022 11:17:14 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Da Xue <da@...re.computer>
Subject: Re: [PATCH] spi: meson-spicc: save pow2 datarate between messages

On 09/08/2022 17:27, Mark Brown wrote:
> On Tue, Aug 09, 2022 at 05:20:19PM +0200, Neil Armstrong wrote:
>> At the end of a message, the HW gets a reset in meson_spicc_unprepare_transfer(),
>> this resets the SPICC_CONREG register and notably the value set by the
>> Common Clock Framework.
> 
>> This saves the datarate dividor value between message to keep the last
>> set value by the Common Clock Framework.
> 
> When you say the value set by the clock framework does that mean that
> the clock driver is adjusting hardware inside the SPI controller IP
> block which is then getting reset by the SPI driver without the SPI
> driver knowing about it?  That seems like a bad idea as you're finding
> here.

The SPI driver is explicitely triggering a reset at the end of each message
to get back to a clean HW state, but it does reset the content of the "legacy"
registers containing the power of 2 divider value, the new registers configuring
the new clock divider path (only on newer SoCs) doesn't get cleared.

> 
>> This didn't appear before commit 3e0cf4d3fc29 ("spi: meson-spicc: add a linear clock divider support")
>> because we recalculated and wrote the rate for each xfer.
> 
> Note that the rate might change per transfer.

It's taken in account, this case is when the rate doesn't change.