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

From: Jan Beulich <jbeulich@suse.com>
To: Roman Skakun <rm.skakun@gmail.com>
Cc: Andrii Anisov <andrii_anisov@epam.com>,
	Roman Skakun <roman_skakun@epam.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Maciej W. Rozycki" <macro@orcam.me.uk>,
	Muchun Song <songmuchun@bytedance.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Mike Rapoport <rppt@kernel.org>, Will Deacon <will@kernel.org>,
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	linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	iommu <iommu@lists.linux-foundation.org>
Subject: Re: [PATCH] swiotlb: set IO TLB segment size via cmdline
Date: Wed, 15 Sep 2021 15:49:52 +0200	[thread overview]
Message-ID: <84ef7ff7-2c9c-113a-4a2c-cef54a6ded51@suse.com> (raw)
In-Reply-To: <CADu_u-Ou08tMFm5xU871ae8ct+2YOuvn4rQ=83CMTbg2bx87Pg@mail.gmail.com>

On 15.09.2021 15:37, Roman Skakun wrote:
>>> From: Roman Skakun <roman_skakun@epam.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