netdev - Re: [PATCH] stmmac: disable tx coalescing

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Message-ID: <20161211190750.GA14287@amd>
Date:   Sun, 11 Dec 2016 20:07:50 +0100
From:   Pavel Machek <pavel@....cz>
To:     David Miller <davem@...emloft.net>
Cc:     peppe.cavallaro@...com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, alexandre.torgue@...com,
        LinoSanfilippo@....de
Subject: Re: [PATCH] stmmac: disable tx coalescing

Hi!

David, ping? Can I get you to apply this one?

As you noticed, tx coalescing is completely broken in that driver, and
not easy to repair. This is simplest way to disable it. It can still
be re-enabled from userspace, so code can be fixed in future.

Best regards,
								Pavel

> 
> Tx coalescing in stmmac is broken in more than one way, so disable it
> for now.
>     
> First, low-res timers have resolution down to one per second. It is
> not acceptable to delay transmits for 40msec, and certainly not
> acceptable to delay them for 1000msec.
>     
> Second, the logic is wrong:
>     
>     if (likely(priv->tx_coal_frames > priv->tx_count_frames))
>      	mod_timer(&priv->txtimer,
>      	          STMMAC_COAL_TIMER(priv->tx_coal_timer));
>     ...
>     
> doing tx_clean() after set number of packets, or set time after the
> first packet is transmitted would make sense. But that's not what the
> code does. As long as packets are being transmitted, you move the
> timer into the future.. so that finally you run out of the place, then
> wait for timer (!) and only then you do the cleaning.
> 
> Third, tx_cleanup is not safe to call from interrupt (tx_lock is not
> irqsave), but that's exactly what coalescing code does.
> 
> Signed-off-by: Pavel Machek <pavel@...x.de>
> 
> ---
> 
> This is stable candidate, afaict. It is broken in 4.4, too.
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/common.h b/drivers/net/ethernet/stmicro/stmmac/common.h
> index 3ced2e1..32ce148 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/common.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/common.h
> @@ -244,11 +244,11 @@ struct stmmac_extra_stats {
>  /* Max/Min RI Watchdog Timer count value */
>  #define MAX_DMA_RIWT		0xff
>  #define MIN_DMA_RIWT		0x20
> -/* Tx coalesce parameters */
> +/* Tx coalesce parameters. Set STMMAC_TX_FRAMES to 0 to disable coalescing. */
>  #define STMMAC_COAL_TX_TIMER	40000
>  #define STMMAC_MAX_COAL_TX_TICK	100000
>  #define STMMAC_TX_MAX_FRAMES	256
> -#define STMMAC_TX_FRAMES	64
> +#define STMMAC_TX_FRAMES	0
>  
>  /* Rx IPC status */
>  enum rx_frame_status {
> 



-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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