linux-kernel - Re: [PATCH v2 2/2] soc: mediatek: add a fixed wait for SRAM stable

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Message-ID: <1525072080.12322.212.camel@mtkswgap22>
Date:   Mon, 30 Apr 2018 15:08:00 +0800
From:   Sean Wang <sean.wang@...iatek.com>
To:     Matthias Brugger <matthias.bgg@...il.com>
CC:     <rjw@...ysocki.net>, <khilman@...libre.com>,
        <ulf.hansson@...aro.org>, <linux-mediatek@...ts.infradead.org>,
        <linux-pm@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, "Weiyi Lu" <weiyi.lu@...iatek.com>
Subject: Re: [PATCH v2 2/2] soc: mediatek: add a fixed wait for SRAM stable

On Fri, 2018-04-27 at 11:46 +0200, Matthias Brugger wrote:
> Hi Sean,
> 
> On 04/23/2018 11:39 AM, Sean Wang wrote:
> > On Mon, 2018-04-23 at 11:31 +0200, Matthias Brugger wrote:
> >>
> >> On 04/23/2018 10:36 AM, sean.wang@...iatek.com wrote:
> >>> From: Sean Wang <sean.wang@...iatek.com>
> >>>
> >>> MT7622_POWER_DOMAIN_WB doesn't send an ACK when its managed SRAM becomes
> >>> stable, which is not like the behavior the other power domains should
> >>> have. Therefore, it's necessary for such a power domain to have a fixed
> >>> and well-predefined duration to wait until its managed SRAM can be allowed
> >>> to access by all functions running on the top.
> >>>
> >>> v1 -> v2:
> >>>  - use MTK_SCPD_FWAIT_SRAM flag as an indication requiring force waiting.
> >>>
> >>> Signed-off-by: Sean Wang <sean.wang@...iatek.com>
> >>> Cc: Matthias Brugger <matthias.bgg@...il.com>
> >>> Cc: Ulf Hansson <ulf.hansson@...aro.org>
> >>> Cc: Weiyi Lu <weiyi.lu@...iatek.com>
> >>> ---
> >>>  drivers/soc/mediatek/mtk-scpsys.c | 24 ++++++++++++++++++------
> >>>  1 file changed, 18 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/drivers/soc/mediatek/mtk-scpsys.c b/drivers/soc/mediatek/mtk-scpsys.c
> >>> index b1b45e4..d4f1a63 100644
> >>> --- a/drivers/soc/mediatek/mtk-scpsys.c
> >>> +++ b/drivers/soc/mediatek/mtk-scpsys.c
> >>> @@ -32,6 +32,7 @@
> >>>  #define MTK_POLL_TIMEOUT    (jiffies_to_usecs(HZ))
> >>>  
> >>>  #define MTK_SCPD_ACTIVE_WAKEUP		BIT(0)
> >>> +#define MTK_SCPD_FWAIT_SRAM		BIT(1)
> >>>  #define MTK_SCPD_CAPS(_scpd, _x)	((_scpd)->data->caps & (_x))
> >>>  
> >>>  #define SPM_VDE_PWR_CON			0x0210
> >>> @@ -237,11 +238,22 @@ static int scpsys_power_on(struct generic_pm_domain *genpd)
> >>>  	val &= ~scpd->data->sram_pdn_bits;
> >>>  	writel(val, ctl_addr);
> >>>  
> >>> -	/* wait until SRAM_PDN_ACK all 0 */
> >>> -	ret = readl_poll_timeout(ctl_addr, tmp, (tmp & pdn_ack) == 0,
> >>> -				 MTK_POLL_DELAY_US, MTK_POLL_TIMEOUT);
> >>> -	if (ret < 0)
> >>> -		goto err_pwr_ack;
> >>> +	/* Either wait until SRAM_PDN_ACK all 0 or have a force wait */
> >>> +	if (!MTK_SCPD_CAPS(scpd, MTK_SCPD_FWAIT_SRAM)) {
> 
> After having another look on the patch, could you change the order of the if:
> So that we check for the existence of the MTK_SCPD_FWAIT_SRAM and sleep and in
> the else branch we to the readl_poll_timeout.
> 
> I think in the future this will make the code easier to understand as you can
> easily oversee the '!' negation in the if.
> 
> Regards,
> Matthias
> 

Initial thought on the patch is that I would like to save a branch
instruction for a most possibly executed block. Or would it be better to
add a compiler to branch prediction information? something like that  

        /* Either wait until SRAM_PDN_ACK all 0 or have a force wait */
        if (unlikely(MTK_SCPD_CAPS(scpd, MTK_SCPD_FWAIT_SRAM))) {
                /*
                 * Currently, MTK_SCPD_FWAIT_SRAM is necessary only for
                 * MT7622_POWER_DOMAIN_WB and thus just a trivial setup
is
                 * applied here.
                 */
                usleep_range(12000, 12100);
...
 


> 
> >>> +		ret = readl_poll_timeout(ctl_addr, tmp, (tmp & pdn_ack) == 0,
> >>> +					 MTK_POLL_DELAY_US, MTK_POLL_TIMEOUT);
> >>> +		if (ret < 0)
> >>> +			goto err_pwr_ack;
> >>> +	} else {
> >>> +		/*
> >>> +		 * Currently, MTK_SCPD_FWAIT_SRAM is necessary only for
> >>> +		 * MT7622_POWER_DOMAIN_WB and thus just a trivial setup is
> >>> +		 * applied here. If there're more domains which need to force
> >>> +		 * waiting for its own pre-defined value, the duration should
> >>> +		 * be coded in the caps field.
> >>> +		 */
> >>
> >> I would say, if necessary in the future we can add a switch statement here.
> >> Other then that the patches look good. If you are OK, I'll just delete the last
> >> sentence when applying the patch.
> >>
> > 
> > yes, it's okay for me. 
> > 
> >> Regards,
> >> Matthias
> >>
> >>> +		usleep_range(12000, 12100);
> >>> +	};
> >>>  
> >>>  	if (scpd->data->bus_prot_mask) {
> >>>  		ret = mtk_infracfg_clear_bus_protection(scp->infracfg,
> >>> @@ -785,7 +797,7 @@ static const struct scp_domain_data scp_domain_data_mt7622[] = {
> >>>  		.sram_pdn_ack_bits = 0,
> >>>  		.clk_id = {CLK_NONE},
> >>>  		.bus_prot_mask = MT7622_TOP_AXI_PROT_EN_WB,
> >>> -		.caps = MTK_SCPD_ACTIVE_WAKEUP,
> >>> +		.caps = MTK_SCPD_ACTIVE_WAKEUP | MTK_SCPD_FWAIT_SRAM,
> >>>  	},
> >>>  };
> >>>  
> >>>
> > 
> >