linux-kernel - Re: [PATCH v1 1/2] scsi: ufs: Introduce device quirk "DELAY_AFTER

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Date:   Wed, 29 Jul 2020 14:17:18 +0800
From:   Stanley Chu <stanley.chu@...iatek.com>
To:     Can Guo <cang@...eaurora.org>
CC:     <linux-scsi@...r.kernel.org>, <martin.petersen@...cle.com>,
        <avri.altman@....com>, <alim.akhtar@...sung.com>,
        <jejb@...ux.ibm.com>, <beanhuo@...ron.com>,
        <asutoshd@...eaurora.org>, <matthias.bgg@...il.com>,
        <bvanassche@....org>, <linux-mediatek@...ts.infradead.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kuohong.wang@...iatek.com>,
        <peter.wang@...iatek.com>, <chun-hung.wu@...iatek.com>,
        <andy.teng@...iatek.com>, <chaotian.jing@...iatek.com>,
        <cc.chou@...iatek.com>
Subject: Re: [PATCH v1 1/2] scsi: ufs: Introduce device quirk
 "DELAY_AFTER_LPM"

Hi Can,

On Wed, 2020-07-29 at 13:31 +0800, Can Guo wrote:
> Hi Stanley,
> 
> On 2020-07-29 13:18, Stanley Chu wrote:
> > Some UFS devices require delay after VCC power rail is turned-off.
> > Introduce a device quirk "DELAY_AFTER_LPM" to add 5ms delays after
> > VCC power-off during suspend flow.
> > 
> 
> Just curious, I can understand if you want to add some delays before
> turnning off VCC/VCCQ/VCCQ2, but what is the delay AFTER turnning
> them off for? I mean the power has been cut by host from PMIC, how
> can the delay benefit the UFS device?
> 

The problem comes from both sides,
1. VCC power rail design by SoC vendors, and
2. Device strategy according to VCC changes

Take Micron UFS devices on MediaTek platform as example, our VCC drop
rate may be too slow and lead to incorrect resume strategy by device.
Without this delay, device may not resume successfully.

Thanks,
Stanley Chu