linux-kernel - Re: [PATCH 1/2] spi/rockchip: fix bug that case spi can't go as fast as slave request

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Date:	Thu, 16 Oct 2014 17:58:51 +0800
From:	addy ke <addy.ke@...k-chips.com>
To:	broonie@...nel.org
CC:	heiko@...ech.de, dianders@...omium.org, grant.likely@...aro.org,
	robh+dt@...nel.org, amstan@...gle.com, sonnyrao@...gle.com,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org, devicetree@...r.kernel.org,
	olof@...om.net, hj@...k-chips.com, kever.yang@...k-chips.com,
	xjq@...k-chips.com, huangtao@...k-chips.com, zyw@...k-chips.com,
	yzq@...k-chips.com, zhenfu.fang@...k-chips.com, cf@...k-chips.com,
	zhangqing@...k-chips.com, hl@...k-chips.com, wei.luo@...k-chips.com
Subject: Re: [PATCH 1/2] spi/rockchip: fix bug that case spi can't go as fast
 as slave request



On 2014/10/16 17:34, Mark Brown wrote:
> On Thu, Oct 16, 2014 at 05:16:02PM +0800, addy ke wrote:
>> On 2014/10/15 21:04, Mark Brown wrote:
>>> On Wed, Oct 15, 2014 at 07:25:49PM +0800, Addy Ke wrote:
> 
>>>> +	if (WARN_ON(rs->speed > MAX_SCLK_OUT))
>>>> +		rs->speed = MAX_SCLK_OUT;
> 
>>>> +	/* the minimum divsor is 2 */
>>>> +	if (rs->max_freq < 2 * rs->speed) {
>>>> +		clk_set_rate(rs->spiclk, 2 * rs->speed);
>>>> +		rs->max_freq = clk_get_rate(rs->spiclk);
>>>> +	}
> 
>>> I'll apply this but you should be checking the return code from
>>> clk_set_rate() here, please send a followup patch doing that.  It might
> 
>> If clk_set_rate return error, do I only put dev_warn here or return error value to spi core?
> 
> It'd be better to return an error if we need to set the rate and can't
> do it.
> 
>>> also be worth consdering just setting the rate unconditionally here, it
>>> seems like it should make things simpler.
> 
>> I think we need.
>> If we set the rate unconditionally here, clk_set_rate() will be executed in each spi transfer.
> 
> Is that really such a high cost?
> 
Not high cost, but  I think if the default spi_clk is enough, we do not need to set spi_clk again.

Maybe we can only set spi_clk as (2 * MAX_SCLK_OUT) in probe().



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