linux-kernel - Re: [PATCH] serial: 8250: omap: Fix idling of clocks for unused uarts

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Date:   Thu, 3 May 2018 07:31:41 -0700
From:   Tony Lindgren <tony@...mide.com>
To:     Vignesh R <vigneshr@...com>
Cc:     Peter Hurley <peter@...leysoftware.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-serial@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org, Keerthy <j-keerthy@...com>,
        Matthijs van Duin <matthijsvanduin@...il.com>,
        Sekhar Nori <nsekhar@...com>, Tero Kristo <t-kristo@...com>
Subject: Re: [PATCH] serial: 8250: omap: Fix idling of clocks for unused uarts

* Vignesh R <vigneshr@...com> [180503 10:20]:
> 
> 
> On Wednesday 02 May 2018 10:45 PM, Tony Lindgren wrote:
> > I noticed that unused UARTs won't necessarily idle properly always
> > unless at least one byte tx transfer is done first.
> > 
> > After some debugging I narrowed down the problem to the scr register
> > dma configuration bits that need to be set before softreset for the
> > clocks to idle. Unless we do this, the module clkctrl idlest bits
> > may be set to 1 instead of 3 meaning the clock will never idle and
> > is blocking deeper idle states for the whole domain.
> > 
> > This might be related to the configuration done by the bootloader
> > or kexec booting where certain configurations cause the 8250 or
> > the clkctrl clock to jam in a way where setting of the scr bits
> > and reset is needed to clear it. I've tried diffing the 8250
> > registers for the various modes, but did not see anything specific.
> > So far I've only seen this on omap4 but I'm suspecting this might
> > also happen on the other clkctrl using SoCs considering they
> > already have a quirk enabled for UART_ERRATA_CLOCK_DISABLE.
> > 
> 
> That's interesting! We do have AM437x suspend/resume working without
> this workaround (UARTs on AM437x does not use DMA) and UART IPs clkctrl
> do go to idle state. Seems like a OMAP4 specific issue.

Yeah seems to be omap4 specific. I did not see this yesterday
on my am437x-idk or beagle-x15 after clearing status = "disabled"
for all ports and idling them.

> Comment in omap8250_update_scr() warns not to set these two bits in a
> single register write because this may lead to malfunction. I would
> recommend to split this into two writes.

OK I'll split the write into two parts.

Regards,

Tony