linux-kernel - Re: [PATCH 08/10] ARM: dts: exynos: use proper 'dma-channels/requests' properties

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Message-ID: <99c80fb6-c6cc-9370-b93d-ed736c7f2192@samsung.com>
Date:   Thu, 28 Apr 2022 12:05:33 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Michal Simek <michal.simek@...inx.com>,
        Liviu Dudau <liviu.dudau@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Broadcom Kernel Team <bcm-kernel-feedback-list@...adcom.com>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Rob Herring <robh@...nel.org>
Subject: Re: [PATCH 08/10] ARM: dts: exynos: use proper
 'dma-channels/requests' properties

On 28.04.2022 12:00, Krzysztof Kozlowski wrote:
> On 28/04/2022 11:57, Marek Szyprowski wrote:
>> On 28.04.2022 11:54, Krzysztof Kozlowski wrote:
>>> On 28/04/2022 11:50, Marek Szyprowski wrote:
>>>> On 27.04.2022 17:58, Krzysztof Kozlowski wrote:
>>>>> pl330 DMA controller bindings documented 'dma-channels' and
>>>>> 'dma-requests' properties (without leading hash sign), so fix the DTS to
>>>>> match the bindings.
>>>>>
>>>>> Reported-by: Rob Herring <robh@...nel.org>
>>>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>>>> Are those properties really needed for PL330 driver on Exynos SoCs? I've
>>>> removed them and I still see the proper values read from registers and
>>>> reported in the log (Exynos4210):
>>>>
>>>> dma-pl330 12680000.dma-controller: Loaded driver for PL330 DMAC-141330
>>>> dma-pl330 12680000.dma-controller:       DBUFF-32x4bytes Num_Chans-8
>>>> Num_Peri-32 Num_Events-32
>>>> dma-pl330 12690000.dma-controller: Loaded driver for PL330 DMAC-141330
>>>> dma-pl330 12690000.dma-controller:       DBUFF-32x4bytes Num_Chans-8
>>>> Num_Peri-32 Num_Events-32
>>>> dma-pl330 12850000.dma-controller: Loaded driver for PL330 DMAC-141330
>>>> dma-pl330 12850000.dma-controller:       DBUFF-64x8bytes Num_Chans-8
>>>> Num_Peri-1 Num_Events-32
>>>>
>>>> I also don't see any code that would read those properties. IMHO they
>>>> should be simply removed at all, at least for the PL330 related nodes.
>>> In current Linux implementation they indeed are not used. Nothing parses
>>> them. However:
>>> 1. They describe (hopefully correct) the hardware.
>>> 2. They might be used by other implementations of pl330 driver.
>>>
>>> I would not remove them from existing sources, but indeed maybe there is
>>> no need to add for new files.
>> What's the point in having dt properties duplicating data that might be
>> read from the driver registers?
> Hm, indeed, there is no point in this. Since they are read from
> registers, what was the idea behind in commit 42cf20980cde?

#dma-cells is indeed required, but the rest seems to be the cargo-cult 
of some kind.


Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland