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Date:   Thu, 9 Mar 2023 09:30:00 +0200
From:   Tony Lindgren <tony@atomide.com>
To:     Yegor Yefremov <yegorslists@googlemail.com>
Cc:     Linux-OMAP <linux-omap@vger.kernel.org>, Bin Liu <b-liu@ti.com>,
        Johan Hovold <johan@kernel.org>
Subject: Re: am335x: performnce issues with FTDI and LOW_LATENCY
Message-ID: <20230309073000.GG7501@atomide.com>
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* Yegor Yefremov <yegorslists@googlemail.com> [230307 09:53]:
> On Mon, Mar 6, 2023 at 8:42 AM Tony Lindgren <tony@atomide.com> wrote:
> >
> > * Yegor Yefremov <yegorslists@googlemail.com> [230228 08:01]:
> > > Any idea why the performance drop is so big?
> >
> > Maybe lots of interrupts and dma not being used for musb in this case?
> 
> Using "irqtop -d 1", I get the following results:
> 
> 3.18.1 LATENCY_OFF (16 ports): ca. 1000 IRQs/s INTC 17 47400000.dma-controller
> 3.18.1 LATENCY_ON (16 ports): ca. 4000 IRQs/s INTC 17 47400000.dma-controller
> 
> 6.2.1 LATENCY_OFF (16 ports): ca. 300 IRQs/s INTC 17 47400000.dma-controller
> 6.2.1 LATENCY_ON (16 ports): ca. 1000 IRQs/s INTC 17 47400000.dma-controller

Hmm I wonder what's causing that. Earlier the Ethernet gadget had some
alignment define tweak that made transfers faster. What kind of data
transfer are you testing with?

> #zcat /proc/config.gz | grep CPP
> CONFIG_USB_TI_CPPI41_DMA=y
> CONFIG_TI_CPPI41=y

>From what I recall musb still handles short transfers with PIO, I think
this is the case also for cppi41 dma. Sounds like that does not explain
the difference you're seeing between 3.18 and 6.2 kernels though.

Regards,

Tony