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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Message-ID: <20200626134151.GA1635@duo.ucw.cz>
Date:   Fri, 26 Jun 2020 15:41:51 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Pali Rohár <pali@...nel.org>
Cc:     Pavel Machek <pavel@...x.de>,
        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

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

Aha, thanks for explanation and sorry for the noise.

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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