linux-kernel - Re: [PATCH 4.19 023/206] PCI: aardvark: Dont blindly enable ASPM L0s and dont write to read-only register

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Date:   Fri, 26 Jun 2020 15:23:22 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Tomasz Maciej Nowak <tmn505@...il.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 023/206] PCI: aardvark: Dont blindly enable ASPM L0s
 and dont write to read-only register

Hello!

On Friday 26 June 2020 14:53:50 Pavel Machek wrote:
> Hi!
> 
> > [ Upstream commit 90c6cb4a355e7befcb557d217d1d8b8bd5875a05 ]
> > 
> > Trying to change Link Status register does not have any effect as this
> > is a read-only register. Trying to overwrite bits for Negotiated Link
> > Width does not make sense.
> 
> I don't quite get it. This says register is read only...
> 
> > In future proper change of link width can be done via Lane Count Select
> > bits in PCIe Control 0 register.
> > 
> > Trying to unconditionally enable ASPM L0s via ASPM Control bits in Link
> > Control register is wrong. There should be at least some detection if
> > endpoint supports L0s as isn't mandatory.
> 
> ....and this says it is wrong to set the bits as ASPM L0 is not
> mandatory.

Negotiated Link Width is in read-only 16bit Link Status register.

ASPM Control bits are in read-write 16bit Link Control register.

> > +++ b/drivers/pci/controller/pci-aardvark.c
> > @@ -321,10 +321,6 @@ static void advk_pcie_setup_hw(struct advk_pcie *pcie)
> >  
> >  	advk_pcie_wait_for_link(pcie);
> >  
> > -	reg = PCIE_CORE_LINK_L0S_ENTRY |
> > -		(1 << PCIE_CORE_LINK_WIDTH_SHIFT);
> > -	advk_writel(pcie, reg, PCIE_CORE_LINK_CTRL_STAT_REG);
> > -
> >  	reg = advk_readl(pcie, PCIE_CORE_CMD_STATUS_REG);
> >  	reg |= PCIE_CORE_CMD_MEM_ACCESS_EN |
> >  		PCIE_CORE_CMD_IO_ACCESS_EN |
> 
> ...but we only do single write.
> 
> So which is it?

That single write was via 32bit memory access which tried to overwrite
both registers (Link Status and Link Control) at the same time.

> Best regards,
> 									Pavel
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html