linux-kernel - Re: [PATCH] dmaengine: pl300: enable the clock to PL330 dma

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Date:	Mon, 4 May 2015 22:50:54 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Dinh Nguyen <dinguyen@...nsource.altera.com>
Cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	dan.j.williams@...el.com, dinh.linux@...il.com,
	vinod.koul@...el.com, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: pl300: enable the clock to PL330 dma

2015-05-04 22:28 GMT+09:00 Dinh Nguyen <dinguyen@...nsource.altera.com>:
> Hi Krzystof,
>
> On 5/4/15 12:30 AM, Krzysztof Kozlowski wrote:
>> 2015-05-04 13:28 GMT+09:00  <dinguyen@...nsource.altera.com>:
>>> From: Dinh Nguyen <dinguyen@...nsource.altera.com>
>>>
>>> Turn on the clock to the PL330 DMA if there is a clock node provided.
>>
>> Why? There is no explanation in the patch for this important question - why?
>>
>> Amba bus already does this and provide a wrapper function.
>> Additionally that would mess up with runtime PM and clock
>> enable/disable.
>
> I don't see the clock for the DMA getting turned on at all, which is why
> after the kernel has booted, the filesystem tries to open up a serial
> port using DMA and the system hangs. The failure is seen here:
>
> http://arm-soc.lixom.net/bootlogs/next/next-20150504/socfpga-arm-multi_v7_defconfig.html

Thanks!

The amba bus and pl330 should enable the clock and then disable it
after probing:
static int amba_probe(struct device *dev)
{
...
        ret = amba_get_enable_pclk(pcdev);
...

I wonder why do you think it is not enabled at all?

>
> This only happens with the multi_v7_defconfig, because the PL330 DMA is
> getting built into the kernel, while the socfpga_defconfig does not
> enable the PL330.

It makes sense. If pl330 driver is not enabled then necessary clocks
are turned on by bootloader. Probing pl330 effectively disables the
clock (if DMA is not used).

> The DTS for the socfpga platform looks like this:
>
> pdma: pdma@...01000 {
>         compatible = "arm,pl330", "arm,primecell";
>         reg = <0xffe01000 0x1000>;
>         interrupts = <0 104 4>,
>                     <0 105 4>,
>                 ...
>                 #dma-cells = <1>;
>                 #dma-channels = <8>;
>                 #dma-requests = <32>;
>                 clocks = <&l4_main_clk>;
>                 clock-names = "apb_pclk";
> };
>
> Perhaps I have the wrong designation for clock-names and the amba bus is
> not able to pick up the correct clock?

I have two ideas:
1. Is this really the clock for the DMA? If DMA is not used then
disabling it should be OK.
2. Disabling the clock may effectively disable its parent or
grandparent if there are not more users. Maybe some other driver needs
these parents to be enabled? This was the issue for at least one
similar error (on Exynos boards).

Best regards,
Krzysztof
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