linux-kernel - Re: [PATCH 1/2] spi: Add Renesas R-Car RPC SPI controller driver

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Date:   Tue, 20 Nov 2018 14:35:30 +0100
From:   Marek Vasut <marek.vasut@...il.com>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     masonccyang@...c.com.tw, broonie@...nel.org,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Simon Horman <horms@...ge.net.au>, juliensu@...c.com.tw,
        linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-spi@...r.kernel.org, tpiepho@...inj.com,
        zhengxunli@...c.com.tw
Subject: Re: [PATCH 1/2] spi: Add Renesas R-Car RPC SPI controller driver

On 11/20/2018 02:32 PM, Boris Brezillon wrote:
> On Tue, 20 Nov 2018 14:09:05 +0100
> Marek Vasut <marek.vasut@...il.com> wrote:
> 
>> On 11/20/2018 08:23 AM, masonccyang@...c.com.tw wrote:
>>> Hi Marek,  
>>
>> Hi,
>>
>>>> Marek Vasut <marek.vasut@...il.com>
>>>> 2018/11/19 下午 10:12
>>>>
>>>> To
>>>>  
>>>>> +
>>>>> +static int rpc_spi_set_freq(struct rpc_spi *rpc, unsigned long freq)
>>>>> +{
>>>>> +   int ret;
>>>>> +
>>>>> +   if (rpc->cur_speed_hz == freq)
>>>>> +      return 0;
>>>>> +
>>>>> +   clk_disable_unprepare(rpc->clk_rpc);
>>>>> +   ret = clk_set_rate(rpc->clk_rpc, freq);
>>>>> +   if (ret)
>>>>> +      return ret;
>>>>> +
>>>>> +   ret = clk_prepare_enable(rpc->clk_rpc);
>>>>> +   if (ret)
>>>>> +      return ret;  
>>>>
>>>> Is this clock disable/update/enable really needed ? I'd think that
>>>> clk_set_rate() would handle the rate update correctly.  
>>>
>>> This is for run time PM mechanism in spi-mem layer and __spi_sync(),
>>> you may refer to another patch [1].
>>>
>>> [1]
>>> https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/commit/?h=for-4.21&id=b942d80b0a394e8ea18fce3b032b4700439e8ca3  
>>
>> I think Geert commented on the clock topic, so let's move it there.
>> Disabling and enabling clock to change their rate looks real odd to me.
> 
> Look at the CLK_SET_RATE_GATE definition and its users and you'll see
> it's not unusual to have such constraints on clks. Maybe your HW does
> not have such constraints, but it's not particularly odd to do that
> (though it could probably be automated by the clk framework somehow).

I think you stated my concern right at the end, good, no need for me to
add to this. Yes, I don't think any random driver should deal with
peculiarities of the clock controller.

-- 
Best regards,
Marek Vasut