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>,
	xen-devel@lists.xenproject.org, linux-mips@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org
Subject: Re: [PATCH] swiotlb: set IO TLB segment size via cmdline
Date: Tue, 14 Sep 2021 17:29:07 +0200	[thread overview]
Message-ID: <7c04db79-7de1-93ff-0908-9bad60a287b9@suse.com> (raw)
In-Reply-To: <20210914151016.3174924-1-Roman_Skakun@epam.com>

On 14.09.2021 17:10, 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? (As an aside,
the bottom of this page is also confusing, as following "Then we can
confirm the modified swiotlb size in the boot log:" there is a log
fragment showing the same original size of 64Mb.

> --- a/arch/mips/cavium-octeon/dma-octeon.c
> +++ b/arch/mips/cavium-octeon/dma-octeon.c
> @@ -237,7 +237,7 @@ void __init plat_swiotlb_setup(void)
>  		swiotlbsize = 64 * (1<<20);
>  #endif
>  	swiotlb_nslabs = swiotlbsize >> IO_TLB_SHIFT;
> -	swiotlb_nslabs = ALIGN(swiotlb_nslabs, IO_TLB_SEGSIZE);
> +	swiotlb_nslabs = ALIGN(swiotlb_nslabs, swiotlb_io_seg_size());

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.

> @@ -81,15 +86,30 @@ static unsigned int max_segment;
>  static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
>  
>  static int __init
> -setup_io_tlb_npages(char *str)
> +setup_io_tlb_params(char *str)
>  {
> +	unsigned long tmp;
> +
>  	if (isdigit(*str)) {
> -		/* avoid tail segment of size < IO_TLB_SEGSIZE */
> -		default_nslabs =
> -			ALIGN(simple_strtoul(str, &str, 0), IO_TLB_SEGSIZE);
> +		default_nslabs = simple_strtoul(str, &str, 0);
>  	}
>  	if (*str == ',')
>  		++str;
> +
> +	/* get max IO TLB segment size */
> +	if (isdigit(*str)) {
> +		tmp = simple_strtoul(str, &str, 0);
> +		if (tmp)
> +			io_tlb_seg_size = ALIGN(tmp, IO_TLB_SEGSIZE);

From all I can tell io_tlb_seg_size wants to be a power of 2. Merely
aligning to a multiple of IO_TLB_SEGSIZE isn't going to be enough.

Jan