Re: [PATCH 0/7] Raspberry Pi spi0 improvements

From: Eric Anholt <eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org>
To: Lukas Wunner <lukas-JFq808J9C/izQB+pC5nmwQ@public.gmane.org>,
	Florian Fainelli
	<f.fainelli-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Noralf Tronnes <noralf-L59+Z2yzLopAfugRpC6u6w@public.gmane.org>,
	linux-spi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Mathias Duckeck
	<m.duckeck-XB/JSsFECOqzQB+pC5nmwQ@public.gmane.org>,
	Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	linux-rpi-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org,
	Frank Pavlic <f.pavlic-XB/JSsFECOqzQB+pC5nmwQ@public.gmane.org>
Subject: Re: [PATCH 0/7] Raspberry Pi spi0 improvements
Date: Thu, 15 Nov 2018 21:11:01 -0800	[thread overview]
Message-ID: <87r2flli8q.fsf@anholt.net> (raw)
In-Reply-To: <20181114055121.5xpwxcu6a5qsgjqv-JFq808J9C/izQB+pC5nmwQ@public.gmane.org>

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Lukas Wunner <lukas-JFq808J9C/izQB+pC5nmwQ@public.gmane.org> writes:

> On Tue, Nov 13, 2018 at 09:12:01PM -0800, Florian Fainelli wrote:
>> On 11/7/2018 11:06 PM, Lukas Wunner wrote:
>> > Here's a first batch of improvements for the spi0 master on the
>> > Raspberry Pi.  The meat of the series is in its last two patches:
>> > 
>> > * Patch [6/7] allows DMA for transfer buffers starting at an offset not a
>> >   multiple of 4.  This overcomes a limitation affecting Ethernet drivers
>> >   such as ks8851 which call netdev_alloc_skb_ip_align() to allocate
>> >   deliberately unaligned receive buffers.
>> > 
>> > * Patch [7/7] speeds up PIO transfers by not polling the RX FIFO when it
>> >   is known to contain data, or the TX FIFO when it is known to have free
>> >   space.
>> 
>> Do you have some performance numbers that you could share before/after,
>> e.g: transfer latencies, number of interrupts and pure throughput?
>
> The throughput is primarily determined by the serial clock configured in
> the DT for a specific SPI slave.  There's nothing we can improve there
> in software.
>
> This series is about reducing CPU usage.  E.g. without patch [6/7],
> transfer buffers not aligned to 32-bit are transmitted with programmed I/O
> instead of DMA.  Thus, constantly receiving packets on a ks8851 Ethernet
> chip with a serial clock of 20 MHz occupies 25% of a CPU on a RasPi 3
> (for the ks8851 IRQ thread).  With the patch, it drops to a negligible
> percentage.  The spi-bcm2835.c driver currently forces PIO even for
> kmalloc'ed buffers which are always contiguous in physical memory,
> i.e. for no reason at all.

With the whole series, I got an improvement in kmscube on my hx8357d spi
panel and vc4 rendering from 5.9fps to 6.6fps.  No stats, but the
numbers seem fairly stable between runs.

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