linux-kernel - Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

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Date:   Thu, 18 Mar 2021 22:34:25 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Jonathan Corbet <corbet@....net>, opendmb@...il.com,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "open list:SWIOTLB SUBSYSTEM" <iommu@...ts.linux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

> > 
> > In fact I should have looked more closely at that myself - checking
> > debugfs on my 4GB arm64 board actually shows io_tlb_nslabs = 0, and
> > indeed we are bypassing initialisation completely and (ab)using
> > SWIOTLB_NO_FORCE to cover it up, so I guess it probably *is* safe now
> > for the noforce option to do the same for itself and save even that one
> > page.
> 
> OK, I can submit a patch that does that. 5.12-rc3 works correctly for me
> here as well and only allocates SWIOTLB when needed which in our case is
> either:
> 
> - we have DRAM at PA >= 4GB
> - we have limited peripherals (Raspberry Pi 4 derivative) that can only
> address the lower 1GB
> 
> Now let's see if we can get ARM 32-bit to match :)

Whatever patch you come up with, if it is against SWIOTLB please base it on top of
devel/for-linus-5.12 in https://git.kernel.org/pub/scm/linux/kernel/git/konrad/swiotlb.git/

Thx
> -- 
> Florian