linux-kernel - Re: [PATCH v2] tty: msm_serial: remove static clk rate setting in probe

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Date:	Wed, 20 Apr 2016 11:36:00 +0530
From:	gpramod@...eaurora.org
To:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-serial@...r.kernel.org, Andy Gross <andy.gross@...aro.org>,
	David Brown <david.brown@...aro.org>,
	Jiri Slaby <jslaby@...e.com>, linux-arm-msm@...r.kernel.org,
	linux-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kernel-owner@...r.kernel.org
Subject: Re: [PATCH v2] tty: msm_serial: remove static clk rate setting in
 probe

On 2016-03-07 08:16, Srinivas Kandagatla wrote:
> The issue with setting up a fixed clock rate at probe is that it would
> overwrite the console rate set by the bootloader for its console 
> device.
> This would result in serial out corruption or missing log when we 
> system
> is booted with earlycon. This is not a issue if we boot system without
> earlycon.
> 
> This setup is at least not required with the mainline driver, this code
> used to be required because the clk_enable() call would fail if
> clk_set_rate() wasn't called first.
> 
> Originally the issue was noticed on DB410c which is based on APQ8016
> chipset.
> 
> Without this patch the console log with earlycon would look like:
> ...
> [    0.000000] RCU: Adjusting geometry for rcu_fanout_leaf=64, 
> nr_cpu_ids=1
> [    0.000000] NR_IRQS:64 nr_irqs:64 0
> ����+HH��0.699378] console [ttyMSM0] enabled
> [    0.699378] console [ttyMSM0] enabled
> [    0.702003] bootconsole [uart0] disabled
> [    0.702003] bootconsole [uart0] disabled
> ...
> 
> with this patch I can see all the skipped lines on the console
> 
> Reviewed-by: Stephen Boyd <sboyd@...eaurora.org>
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
> ---

Tested on DB410C. Fixed below curruption with earlycon:

[    0.815843] msm_iommu_ctx 1e3b000.qcom,iommu-ctx: context periph_SDC2 
using bank 27
����[    1.411080] console [ttyMSM0] enabled
[    1.411080] console [ttyMSM0] enabled

Tested-by: Pramod Gurav <gpramod@...eaurora.org>

> 
> Changes since v1:
> 	- removed empty line suggested by Greg KH
> 	- added reviewed-by
> 	- added extra comment suggested by Stephen Boyd.
> 
>  drivers/tty/serial/msm_serial.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/msm_serial.c 
> b/drivers/tty/serial/msm_serial.c
> index dcde955..3bf41ab 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -1579,8 +1579,6 @@ static int msm_serial_probe(struct 
> platform_device *pdev)
>  		msm_port->pclk = devm_clk_get(&pdev->dev, "iface");
>  		if (IS_ERR(msm_port->pclk))
>  			return PTR_ERR(msm_port->pclk);
> -
> -		clk_set_rate(msm_port->clk, 1843200);
>  	}
> 
>  	port->uartclk = clk_get_rate(msm_port->clk);