linux-kernel - Re: [PATCH] swiotlb: set IO TLB segment size via cmdline

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Message-ID: <84ef7ff7-2c9c-113a-4a2c-cef54a6ded51@suse.com>
Date:   Wed, 15 Sep 2021 15:49:52 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Roman Skakun <rm.skakun@...il.com>
Cc:     Andrii Anisov <andrii_anisov@...m.com>,
        Roman Skakun <roman_skakun@...m.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Muchun Song <songmuchun@...edance.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Mike Rapoport <rppt@...nel.org>, Will Deacon <will@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        iommu <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH] swiotlb: set IO TLB segment size via cmdline

On 15.09.2021 15:37, Roman Skakun wrote:
>>> From: Roman Skakun <roman_skakun@...m.com>
>>>
>>> It is possible when default IO TLB size is not
>>> enough to fit a long buffers as described here [1].
>>>
>>> This patch makes a way to set this parameter
>>> using cmdline instead of recompiling a kernel.
>>>
>>> [1] https://www.xilinx.com/support/answers/72694.html
>>
>>  I'm not convinced the swiotlb use describe there falls under "intended
>>  use" - mapping a 1280x720 framebuffer in a single chunk?
> 
> I had the same issue while mapping DMA chuck ~4MB for gem fb when
> using xen vdispl.
> I got the next log:
> [ 142.030421] rcar-fcp fea2f000.fcp: swiotlb buffer is full (sz:
> 3686400 bytes), total 32768 (slots), used 32 (slots)
> 
> It happened when I tried to map bounce buffer, which has a large size.
> The default size if 128(IO_TLB_SEGSIZE) * 2048(IO_TLB_SHIFT) = 262144
> bytes, but we requested 3686400 bytes.
> When I change IO_TLB_SEGSIZE to 2048. (2048(IO_TLB_SEGSIZE)  *
> 2048(IO_TLB_SHIFT) = 4194304bytes).
> It makes possible to retrieve a bounce buffer for requested size.
> After changing this value, the problem is gone.

But the question remains: Why does the framebuffer need to be mapped
in a single giant chunk?

>>  In order to be sure to catch all uses like this one (including ones
>>  which make it upstream in parallel to yours), I think you will want
>>  to rename the original IO_TLB_SEGSIZE to e.g. IO_TLB_DEFAULT_SEGSIZE.
> 
> I don't understand your point. Can you clarify this?

There's a concrete present example: I have a patch pending adding
another use of IO_TLB_SEGSIZE. This use would need to be replaced
like you do here in several places. The need for the additional
replacement would be quite obvious (from a build failure) if you
renamed the manifest constant. Without renaming, it'll take
someone running into an issue on a live system, which I consider
far worse. This is because a simple re-basing of one of the
patches on top of the other will not point out the need for the
extra replacement, nor would a test build (with both patches in
place).

Jan